Have you ever reloaded after getting a bad end at all? Cos you have to have all the bad ends on the same save/playthrough for the unlock to work :3 If you get a bad end with a CG, then reload an older save and carry on, the CG will stay unlocked in the gallery, but the point from the bad end won't count :(
Also, not all of the bad endings have CGs, so it can be awkward to keep track of those ones.
There is a cheat that you can use as well if you want me to tell you that instead of going back through the game :3