Dentures are held in by the bone and soft tissue. A suction forms between the two, with a small layer of fluid creating a seal. When you lose any or all of your teeth, you also lose bone over time. These changes in size and shape allow for more space to develop between the denture and tissue allowing them to lose their seal. This leads to the denture shifting and rocking, increasing the chance they could fall out.
One way to improve this is to add an adhesive that increases the retention of the denture. This is a very short-term solution. A second solution is to have your dentist do a lab reline of the denture. This increases the adaptation of the denture to the bone and tissue, increasing the seal. Over time this will need to be done again, therefore, is a temporary solution. A long-term solution would be to have some implants put in place that would hold a denture or implant supported crowns.
Implants lock the teeth in place, allowing you to chew at significantly higher forces. They also help hold your bone in place, which helps prevent bone loss over time. Transitioning to an implant supported denture or bridge would allow you to eat and smile more comfortably over a longer period of time.