is it winding too tight? placing pressure on the outside of the bobbin? try slowing way down when you wind it...is it possible to wind them while you control the speed? i only had this problem once...in my case i did not have the thread traveling through the correct places and it wound very very tight. i had to start over, making sure my thread was held on a spindle and 'threaded' through the proper places, and i had to slow way down. mine actually goes through the tension disks and when i had the problem the thread was (on) the disks but not between them like it should have been.
not all monofiliment thread is created equal, some stretches alot...and causes tightening.less speed should help