That is an odd one... Which model?
Perhaps there is a failure inside the hull and the knobs are flexing under load? Take a look inside. There is a column of plastic that the bolts thread into. If the column is sheared, that may allow the knob to flex and release the drum shaft. If not, maybe the earlier slippage made a ramp up the front of the shaft recess? A little re-shaping of the recess should allow the shaft to sit back down under the knobs. If you cannot get that done, a little plastic welding in front (a little wedge) could be added in front of the shaft. Each issue may be warrant-able, but to get use of the hull while waiting, I would try to fix.
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Hobie Cat USA