1) One of the most common causes of a loose upper denture is a lack of seal in the post dam area. To see if this is the case: before embarking on treatment, place some greenstick compound over the post dam area of the upper denture and seat it in place. If retention is improved the likelihood of improving things is high.
2) Combination syndrome describes a situation whereby an upper complete denture opposes natural teeth.
If the tooth set up is such that there is an overbite, during mandibular protrusion, the lower anterior teeth can dislodge the upper denture. If the teeth on the upper denture are moved forward to increase the overjet, the dislodging forces are decreased.