Cuffmerge won't allow me to add all four of my Cufflinks-generated GTF files for merging. Under Additional GTF inputs it says "No gtf dataset collection available". But there are 4 Cufflink assembled GTF files in my history (each with >24,000 lines). I've tried logging out and back in. I've tried deleting the entire history and repeating the data import, Tophat, and Cufflinks (in case sometihng got corrupted aong the way). I'm not sure what else to try and I could use some help.