The first thing I would suggest is that you read the manual and make sure you are following the directions.

The second thing I would suggest is that you take the bottom off and see if your gears are shattered. There will be broken pieces of plastic in the bottom. The screws to remove it are in the feet and in the middle. It's easy to see.

The only T&S that I have left is the one I bought new. Mine has always been stored in the house. It has had light use as well. All the others I've bought or have been given to me have developed broken gears shortly after using them.. The first gear to go is usually the bobbin filling gear.

The 750 has plastic gears. The only ones that don't are the 600s. Some of the 620s have metal gears as well. If you will check to see if you have broken gears and report back that you don't, I'll dig my T&S (776, I think) and see what the problem might be.

It's usually not cost efficient to repair the T&S. You can buy a 400 or 500 series machine for the cost of buying the gears (labor is extra). The only thing a 700 series will do that the earlier ones won't is a stretch stitch and a chain stitch.
Let me know and I'll see if I can duplicate your problem.

bkay.