I have a dec 600 embriodery machine that has stopped picking up the bobbin thread

First suggestion is install a brand new needle, but since you've already done that... Most likely the machine has been knocked out of time, ie the needle is arriving at the bobbin at the wrong time. Search for "sewing machine timing" or take it for service.

Try a brand new needle.

youtube not picking up bobbin thread Google Search

...

Usually, tightening the upper tension will pull the bobbin thread up. Tension is correct when both upper thread and bobbin thread meet in the middle of the fabric.

However, perhaps the bobbin tension is either too loose or defective/broken. Try adjusting the bobbin tension tighter and retest. If it doesn't appear to be making any progress, then service may be your option. Or perhaps picking up a new bobbin case for your machine will be a less expensive fix (BTW, having an extra bobbin case is a good idea).

...

Sound s like top tension too high, bobbin tension too loose, or a needle issue - dull or incorrect one for the fabric.
Change the needle, check tensions.
Have you stabilised the fabric?

hold thread end w/3" length and put the it in the bobbin case and make sure its unwinding clockwise..Feed thread thru the higher left metal notch and pull to lower left notch and make sure the thread stays behind the spring slot and leave about 4"tail...put bobbin in its place lining up the white dots close the case and turn the wheel to drive down needle holding the needle on the machine base losely and draw the bobbin thread up

Hello,

You can take the slide plate off, that your needle goes through and watch to see if the arm grabs the bobbin thread, if you can get it to pick up, than slide scissors or something flat under your needle and manually pull both threads through, replace the plate.

Your bobbin thread could be wound to tight or spliced threads on it, and your tension needs to be adjusted. If your using a pedal, try setting to a slower speed.

I would try a different bobbin and thread it manually as in the step I mentioned, if you can't see the arm grabbing it, you will need to check to see if you have loose threading jammed in youir machine, if this isn't the case and adjusting the tension dosen't help...than you will need to replace the bobbin casing.

Kind regards!

I have a 600 also...have you changed brand of white thread? Sometimes it can be the design you are embroidering that will bring bobbin thread to the top. I use the prewound bobbins--very thin thread & tightly wound.

Try changing the needle again. I have had some cause me trouble even if new. And make sure the bobbin is in the right way.

It's okay - it's supposed to do that! The Needle Down button has a different function when you are in Embroidery mode i.e. you have the emb. unit attached.
It moves the emb. arm forward for two reasons: so you can cut top-side jump stitches more easily between colour changes (you touch the button again and the emb. arm returns to under the needle so you can finish the design); and secondly so you can "park" the embroidery unit when you are finished. You can then turn the machine off, remove the emb. unit and replace it in the carry case. If you DON'T do this, the emb. arm is not in the right position to fit into the moulded carry case properly.

The Broken Thread message usually means that it has started the colour, but the thread has been pulled down underneath instead of making nice locked stitches. Hold onto the thread end until the machine has stopped and beeped for you to cut the thread. This usually solves that problem. Sometimes the thread gets pulled out of your hand so just back up the stitches a few and start over, holding the thread more tightly. Don't tug on the thread - just keep a firm grip.

Designer 1s often cut the bobbin thread too short so that it can't make those nice locked stitches when it starts. You need to take off the hoop, open up the bobbin cover and use your stylus, the tip of a pair of scissors, tweezers (whatever is to hand) to pull the bobbin thread out a little. Instead of cutting it on the bobbin thread cutter, just let it hang outside by 1/8" or so. That way you can be sure there is enough bobbin thread available for the stitch to lock. Don't use the scissors button on the machine after every colour change. Sure, it makes for a tidier back of the embroidery with less work for you afterwards, but if it means you are taking the hoop and bobbin cover off after every colour change then it is not worth it in time-saving.
If your machine is doing this - cutting the bobbin thread too short - mention it to your tech. when you machine goes in for its next service. He may be able to tweak the bobbin cutter so it does not cut so close.

First easy thing to check, are you using the correct length needle.
If this is not the problem then somehow the needle bar has moved and the the needle depth is to high or the timing is wrong..
Unless you really know what you are doing then I am afraid a sewing machine tech needs to reset the needle depth and subsequently the timing.