Loss of sensitivity is actually a common issue when jelqing incorrectly. If a man is using a grip where the fingers, and thumbs right along the top of the penis...or even when stretching...if pressure is being applied on the TOP of the penis, these issues with sensitivity and even in the long term erection problems can develop due to the DORSAL NERVE running along the top of the penis. Remedy is very simple, switch to grips that grasp along the sides of the penis, and jelqing that uses a 'palm down' grip where the thumb and forefinger start pressed against the pubic bone. See this page on WIKI:
Dorsal nerve of the penis - Wikipedia, the free encyclopedia