Most motor skill based movements (think riding a bike, throwing a football, or snatching) are neural patterns built through repetition and skill refinement. Once your body has learned these motor pathways they tend to become repeatable even with a large lapse of time. The ability to move heavier and heavier weights, however, is a process that needs to be consistently trained in order to be kept. The body responds to training stimulus or stress and will adapt to accommodate the work being asked of it. If you have not put a heavy weight on you back or overhead in quite some time you can expect it to feel a whole lot heavier and your general ability to complete these lifts will decrease as time untrained goes by. This is why top lifters will often continue hitting heavy percentage squats leading up to competition week even though the overall volume of training may have decreased. If you feel as though the bar always feels heavy during squats pick a structured 5 week squat cycle to reacclimate the body and joints to the stress. If heavier jerk attempts are being missed due to a weak dip/drive phase up the frequency of jerks from support such as blocks or add in jerk dip squats to simulate the load.