There are several items that can cause issues. please check all of the following:
Check for GMSL link lock (PCLK detect requires an active GMSL2 link lock). Note that for dual and quad deserializers, there could be separate link LOCKED register bits for each GMSL link.
Check for MIPI input Phy errors (mismatches in number of lanes, lane swapping, or polarity will show up as phy errors)
Check all input pipes/phys (GMSL serializers may have multiple input phys and video pipes and not all of them are activated/connected by default)
Check that the input PCLK rate is within bounds (PCLK should be less than 300MHz for most GMSL systems)