Quote Originally Posted by dhouston View Post
It appears that after sensing the first high, it never senses a low, so it cannot sense another low-to-high transition.
Thanks Dave you were right. I removed the pull up and in the process found a dry joint. This may have been the source of the issue in the first place, but was causing the intermittent behaviour.

Interrupt is working now and I can continue working through the testing.

Thanks everyone for your help.
Danny