F/- dislodgement can be due to insufficient resistance or retention. Most general dentists think that retention is the most important factor but like in C&B, resistance is infinitely more important than retention. Resistance depends on primary and secondary support areas e.g. palate, tuberosity vestibule etc. Good resistance requires good extension without under- or OVER-EXTENSION. On the other hand, retention depends on your posterior seal e.g. post dam.
I can write a 3 hr essay on this. I will try and keep it brief. I have rated this in order of likelihood.
Assuming impression and acrylic processing is accurate, when the patient opens his/her mouth and denture dislodges. It can be due to,
1.) over-extension in the vestibular areas or over-extension posteriorly more than hard/soft palate junction, more than the palatine fovea and the 'vibrating line'. The way to check it is with a) GC Fitchecker which is a snap set PVS material or b) pressure indicating paste (PIP). With it only needs a very thin layer on all the fitting surface and flanges. While material is setting, you need to muscle trim. Check the labial as well as the 2 buccal frenal attachments. Most of the time, the high attachments dislodges F/-. When the material sets you can visualize along the flanges where the thin PVS material is which indicates over-extension. Thin layer of PVS on the fitting surface indicates inaccuracy - needs adjustment of only those high areas. By enlarge, most problems derive from over-extension. Under-extension of the palate can be a cause but unlikely. I will check it though.
2) very mobile alveolar mucosa in anterior maxilla as a result of significant ridge resorption when patient is edentulous in the maximal but has retained mandibular anterior teeth e.g. combination syndrome. When patient is functioning e.g. occlusal contact -> distortion of soft tissue followed by mouth opens leads to rebound -> dislodge of denture base. If this is the case, your impression technique was incorrect. Requires a retake impression of either a combination of muco-compressive and mucostatic approach or just mucostatic approach. Otherwise, if you want to salvage the newly fabricated F/-, can try to hollow out acrylic where mobile tissue is and do a functional reline impression for 2 days and ask pt to return to perform a cold-cure acrylic reline
3) Insufficient palatal seal. Using fit-checker can help you to visual this. If there is a thick layer of material when you load the F/- in the primary support area e.g. continuous thumb pressure on palate when material is setting and there is a thick layer of PVS material after it has set. You can bet you that the posterior seal is poor, need to score more post dam on the cast before processing or otherwise, reline with green or brown stick.
Hope this helps. Removable Prosthodontics is a lost art that most general dentists do poorly. Without it, they also do not understand the intricacies of tooth positions relating to full-mouth rehabilitation of teeth and/or implants.
Happy to answer other material science and clinical questions from a Prosthodontics Resident's perspective (Board certified, awaiting thesis submission)