We’ve seen this happen twice recently with different customers so thought we should post our solution.  We have seen where a Ubiquiti AP (NanoBeam & NanoStation) will power up via the power injector and the unit is reachable via wireless, but there is no link light on the switch or to a laptop plugged directly in.  In both cases the units were mounted up on a roof, so we first replaced the cables from the injector to the switch but nothing changed.  We found we had to either re-terminate the RJ45 ends on the cable going from the injector up to the roof or replace the cable completely before it started working again.  The cable end or cable itself was damaged so that it wouldn’t allow link, but PoE passed through.