Alistar not getting credit for objectives

Was playing support Alister in a game just now, and even when i was the only one hitting objectives it didn't show me as getting credit for it, it just showed the icon for minions as having destroyed it (same thing happen when teammates where there hitting it, they got credit I didn't). I check the stats and I know I got the last hit on at least one tower, it showed 0 towers and 0 inhibs (Killed the inhibs by myself not sure if you have to get the last hit for it to count in the stats or what). Weird thing is, I did get an S- even though i was only 2/3/15 in a 27 min game so it seems like it was counting them for that (Killed or helped kill 5 turrets, 2 inhibs, and the dragon)