I have experienced a strong odor coming from the vent pipe sometimes but most of the time there is no odor. Why is this?

The weather, especially barometric pressure, can play a big role in the effectiveness of your ventilation system. Most of the time there is some sort of breeze and the air is usually light in nature. This allows the odors to escape from the vent pipe pretty much unnoticed. When the air is heavy and humid with little in the way of a breeze, those same odors cannot rise into the atmosphere as they would normally, so they tend to hang in an invisible cloud form near to the point of release from the vent. That is when they become more apparent and bothersome. There is really nothing wrong with your vent system, the odors simply are not allowed to escape and there is not any magic remedy for this problem, other than to try to mask the odor with chemicals which can be flushed through the system. This however is very short term, and does not really fix the problem.