Some times the rear springs is just a bit twisted to the sides, and the abar doesn't hit all the way flat, leaving a small air gap on one of the sides coil core. To check this bring the armature towards the front coil while while you look at it through a bright light. Press slowly and see if the contact is perfect. Rotate the machine all around to see if you have any small air gap.
It could also be that the air gap is either at the top or bottom of you top coil meaning that you need to shim down or up.
Very low compression and excessive voltage can cause this problem too.
If your springs are old change them! there is nothing better than fresh springs.
Make sure you have a friend or someone that can tune the machine properly if you don't know how to do it.
Almost every machine, including shitty ones can be tune to have a nice sound and feel. In most cases that issue you are experiencing relies just on, springs, shimming and tuning, not in the machine itself. Unless you got yourself some really but really bad cheap machine.
Gaston - Fallen King Irons