There are many possibilities as to why you do not feel the urge.
The most likely reason is that you have not paid attention in the past to the urge to defecate and so your body has given up trying to tell you to go to the bathroom. If this is the issue, you need to let your body know that you are ready to listen again. One way to do this is through bowel training which is just like toilet training for the youngsters-you go to the bathroom after every meal and every 2 hours. When you start evacuating well after every meal you can let go of the every 2 hours. Please never push it out. This is very hard on your cardiovascular system-Elvis comes to mind. Gentler methods such as abdominal effluerage or acupressure to various points such as stomach 36 or pericardium 6 are very helpful.
Other reasons are variations in nervous system stimulation. The first place to look is in your family. Has anyone in your family had any history of this issue or of neurological changes?