Generally speaking, when timpani pedals won't stay in place it is due to the base pitch of the drum not being correct. With the pedal all the way down the drums should be:

32" = D
29" = F
26" = A
23" = C

If this is correct the pedals should stay in place. If not, it will require additional adjustments, but I would HIGHLY RECOMMEND not turning knobs or messing with other adjustments if not necessary.