Question about All Pick Ban.

In Unranked All Pick, why doesn't ban work sometimes? The first 5 seconds are to pick a hero to ban, but sometimes it simply won't let me ban. I always try to ban sniper because I think he's OP as shit, but it really doesn't let me like a third of the time.

Also, is there a resource that is just like a dictionary of all the abbreviations and random words people use when talking about DotA? I could make a list of the terms I don't get, but I figure someone has to have done this already.