missing transponders

I've got aVu+ Solo4k & Duo2 connected to a 1.1m motorised dish, both running OpenVix 5.0, I've not been on 16 east for some time, my old channel list was out of date, so did a manual scan, but strangely several transponders don't show, my 1.1m dish isn't very efficient as it's warped, but no...