uplink sniffing

Hi all, Just received my C123 (from http://shop.sysmocom.de/) with filter rework and it works great. I have one problem with the "burst*.dat" files when i do uplink sniffing, basically there are no files produced and when they are produced most of the time they are empty? This doesn't happen when i do downlink sniffing. Is this a stupid question or am I missing something... cheers, L.