If it's on the FTA channels too (BBC 1 + 2 SD, ITV 1 SD, Channel 4 SD and 5 SD) then it's likely a signal issue.
If you're using a splitter, make sure you use a quality 5-1000MHz one (for ex. Tratec Ecoline) not a cheap no-name brand. Also, make sure you're using VM grade cables, not satellite cables (there's a big difference).
If you're still getting problem, try again but this time disconnect every else from the VM socket and connect the H2H to it directly (no splitters).
If it still persists you will either need a signal booster (your signal is too weak) or an attenuator (your signal is too strong). The zgemma boxes are quite delicate when it comes to signal levels - both too high and too low. If you have a VM Superhub for internet, have a look at your line stats - it will give you power levels in there. Target power levels are 0dBmV - anything positive is above target and anything below is under target. Typically I've found the zgemma is best between -4 dBmV and +3 dBmV.