strange block type thingy

hi, i use AIM and im having a little problem. my friends’ screen name doesnt appear in the “online” section of my buddy list as if he had blocked me. however, i know for a fact that he didnt because he has imed me. the weird thing is…when he imed me, his sn STILL didnt show up on the “online” section. and iknow if he had blocked me, it wouldnt hav been possible for him to iM me. so im a little confused as to why this is happeneing. and i know i didnt type in the screen name wrong because ive had him on my b-list for about a year now lol…this problem just propped up in the last week or so. thanks in advance:)