Ive had intermittent issues with PayPal IPN callbacks. Im not sure if it is a PP thing or ECT. It started about 2 weeks ago and it just goes in and out. Ive received some emails from PP saying that the feature could be disabled from my account if it isnt resolved. If you have any experience with this please let me know.
I know PayPal have been reporting IPN problems recently so that may be part of the problem, but maybe not all. There's a good thread here with things to look at and try which may turn up something with your settings https://www.ecommercetemplates.com/support/topic.asp?TOPIC_ID=105904