Sounds like the bobbin may not be the right one for your machine. YOu haven't said what make and model this is but you should check your manual or online to ensure you have the correct bobbin for it. I've seen Elna's with Singer bobbins in, they'll stitch okay but then tension problems occur, you need to have the right one for your machine. (if you aren't sure, try looking at www.sewusa.com
to find the bobbin described for your machine.)
Also metal bobbins can get squished if they've been stood on, so have a good look at it and make sure the bobbin is in good shape and looks okay.
Now take the thread off it and rewind a fresh bobbin making sure you've got tension on the thread the whole time and that the thread winds onto the bobbin very smoothly and evenly. This is something that is not emphasied in sewing machine manuals but makes a HUGE difference. Think of your garden hose coiled onto the hose reel. If you wind as the hose wants to go, it goes onto the reel all over the place, then jams up and you can't get it all on there. Look at how a thread spool is wound, the factories wind the thread in a very neat sequence and you want your bobbin to look the same. So always use the tensioner on top of the machine to take the thead around when winding the bobbin. If you don't have one, then put tension onto the thread spool the thread is coming off to ensure the thread runs off at a constant tension and speed.
Carefully watch the thread as it goes onto the bobbin and nudge it with your fingertip to get it to fill evenly top, bottom and middle of the bobbin. Stop when 3/4 full.
Now put bobbin to bobbin holder ensuring that it goes in the right way, always the thread should come off the bobbin and reverse direction in a V shape. Click the thread into the tension spring. Double check it is in there and then thread up the rest of the machine and test sew.
THere is some good trouble shooting tips at www.sewusa.com
so this might help you also.