Weakened pelvic floor muscles
Muscles around the bladder help to empty urine from the bladder. If the muscles become weakened, it can cause a varying quantity of urine to remain in the bladder after urinating. If this happens, you soon have to go to the toilet again.
The muscles weaken with increasing age, but it can also be a symptom of a disease that you should discuss with your doctor. Diabetes and vitamin deficiency are a couple of examples. The sooner you can get treatment the better.