I once asked the question "Why is it that I cannot answer a certain person's questions?" and was told it was because that person must have blocked me. I cannot help you further than that I'm afraid. It doesn't take very much to upset someone here and cause them to block you. It isn't worth losing any sleep over though so I'd suggest you just accept that some will block you and then leave it at that and answer those who are happy to hear from you.
If you offended a Man United; Liverpool or Arsenal fan then you have narrowed it down to a few possibilities. These fans are the easiest to offend so when you try to answer a question and it says "privacy in red ink" and won't accept your answer then that's your blocker.
Normally if you request to be their friend, or you want to add them on a network, sometimes it may tell you that you can't because you've been blocked. Other times you won't see their profile, which is another sign that they've blocked you.