With the 6255 the clamp has 5 silicon buffers instead of the usual 4, this adds support, but only if all 5 are in correctly, it is possible for the 5th middle one to be in wrong and prevent the other 4 from fully engaging, check that the 5th one is through the hole and that the lip of all 4 of the others on both ends are fully engaged and locked in, generally if all 5 are in right, unless you are trying for a very sharp angle of nearly 90 degrees or there is inordinate tension on the cord pulling it up, they should be sufficiently strong to hold the pump without issue. The 5th one buts up against the back of the pump so it cannot move easily in the c clip. If you want to lock them in, a trick I have found is the silicon pegs have a small hole in the middle, if you build models or have any plastic pieces that come on those boards where you twist/cut them off the pegs, those small left over peg/skeletons can be cut and pushed into the holes and will lock the pieces together.
