Lower dentures can be difficult to wear and are the source of many headaches with denture wearers. Many dentists shy away from doing dentures altogether due to the common complaints with lower dentures. While most denture wearers adjust to their lower dentures, many simply carry it around in their pocket or leave it on their night stand.
The biggest reason why the lower denture is uncomfortable is because it doesn't "suck" like the upper denture. The shape of the upper jaw and palate allow the upper denture to have some suction and stay in place on the upper palate. The lower jaw is U-shaped and very rarely provides any suction and relies on the bony ridge to secure it. If you have bone loss, which is common with tooth loss, you many not have sufficient enough of a ridge to support your lower denture effectively.
The second reason is that there are more muscles around a lower denture which can move it around side to side. The tongue is right next to the lower denture and is a powerful muscle that has to learn new habits to adapt to it. Also, the cheek muscles and chewing muscles which attach to the lower jaw can lift the denture and push it out of place.