Loose partials. The metal hooks on the partial may need to be tightened, or the gum tissue has receeded and you may need the partial re-lined with a new thin layer of plastic. Ask your dentist.
Poor fit. Either due to lack of supporting bone, loose teeth or a combination of both.
Dentures. Many factors can contribute to that. Including: -loose clasps: they may need to be adjusted -few supporting teeth: you may need dental implants to assist on the partial support. -poor fit: if so, you may need a reline (even if the partial is new) -over-extension of the borders: a simple adjustment can help you here good luck!
Doctor visit. First how old is the partial denture? Second are the supporting teeth and structures healthy n stable? It might be time to see your doctor for adjustments or a new prosthesis. If the supporting structures are healthy n stable adjusting the clasps should take care of the retention problem.
Not uncommon. Partial dentures may fall out due to age and wear or limited retention, i.e. Insufficient number of teeth to hold them in. Partial dentures do not lock in place and stay as if they were attached to your bone. They must move or else you risk losing the teeth the partial attaches too. If this is what you want you need to consider dental implants.