So have you had the tension unit apart and cleaned it?
Theoretically reversing the stitch direction shouldn't make any difference to tension - the tension unit becomes 'live' when the foot is dropped, and releases when the foot is raised, it's a simple internal spring-activated mechanism.
Are you sure the machine is threaded correctly?
Is the thread in the bobbin feeding out in the correct direction?
And it's not an issue of the wrong height for the feed dogs? Just thinking out loud there, but as it is possible to move them, then I guess they could be positioned wrongly and failing to move the fabric as required in the reverse direction?
Sorry I can't get mine out and actually fiddle about, but am about to be away from home for a couple of days.
Hope you can get this sorted
Leila