Dentures are considered the third set of teeth in a person’s life time. When needed, they are considered a bliss, no doubt. Having no teeth has many undesired effects.
Dentures get their retention, that is they stay in place, by adhesion to the gums. This adhesion is helped by a thin film of saliva. In simple words, the dentures stay in place by suction to the gums.
This adhesion is usually sufficient for the top dentures, however, it is another story for the lower dentures.
Lower dentures are usually not as retentive as top dentures for the following reasons:
1- Smaller surface area of gums.
2- Tongue movement encroaches on the dentures and keeps moving them from place.
3- The bone ridge is not as defined in the lower arch.
This site gives you an idea about the overdenture solution for unretentive dentures.