Without knowing more, it sounds like a cable problem. Did you try re-terminating both ends? Is your cable length under the limit for both the CATx and type of POE? Did it get run past something which could be providing interference?
As to why the tester is showing a good cable, all things being equal I'd think either your cable tester isn't testing all of the pairs, isn't working properly, or the cable functions just enough to test out continuity-wise, but not well enough to carry the POE properly to the camera.
It might be worth picking up a POE tester to see what the voltage looks like.
I went through this over the phone for a long time with an installer. He "certified" the cables with a tester, etc.
The ends were terminated in order and not in "pairs".
IPVMU Certified | 05/15/16 03:55pm
Are you using same cable manufacturer and same cable type when making a cable change? What standard was used during certification?
IPVMU Certified | 05/15/16 05:30pm
You should do a full cable test and demand a report. Even than, as stated above, someone might have mqde fake reports so if you want to be sure, test it yourself.
A simple thing as a split pair will not be detected by a 'dumb' cable/wire mapper tester but will certainly stop your link from working.