studio instruments

I have a project that has15 tracks so far 1 SI piano 2 SI drum tracks the rest all audio tracks. I am trying to add another SI drum track but it wont let me, the options are grayed out. I did have a third SI drum track to start out with but I deleted that when it wouldn't let  me add a fourth track. So I combined two drum tracks into 1 making the two I have now but I still  can't add a third SI track, drums. piano ,anything! Any ideas?