When mine does that it generally needs a new needle. It isn't usually the tension for me. Also on a 15, the flat side of the needle goes to the left, just like a Featherweight. The 15 is a very good machine. Get out the manual and DON"T GIVE UP. Rest between, if needed, but your machine just needs a little help. Also, be sure you are using a good 15 bobbin, not a 66. There is a difference and your machine can tell. The 15 is totally flat on top and the 66 has a slightly rounded top. It happens.