Used to happen to me quite a bit until I switched to metal tanks.
The Paterson tanks used to come with a small plastic clip to prevent the reel from lifting up on the black spindle.
Without this clip, the reel can work its way up to where you would need more developer to cover the film.
I'd recommend measuring the amount of liquid needed to completely fill the tank and just assume the reel is going to ride up.
Or switch to metal.