As Helene told already, most Paros tolerate many different hiding places and caves. But most prefer caves that are not too wide. Normally. coconuts are very wide indeed. Our German breeder Bernd Bussler is very successful using extremely narrow caves measuring not more than 1 centimeter in diameter. Even species with large nests (as filamentosus) breed there successfully. But with this species they can be wider too. A coconut however is too wide, normally. Nevertheless, if there is no alternative, they take it.
I once have used in two of my 33 small Paro-tanks (which contain one pair and one cave, very different caves, each) coconuts too, because they look so “natural”. But I did not repeat that because of bad experiences. I was unable to control what happens inside and the male fish obviously did not like the wide room very much. Are you able to have a good look into your coconut? You should in order to explain breeding success or breeding failure. In your case it looks like a failure. But why?
Most species maintain a cave for very long times. In my tanks, they have only one on the bottom and are very reliable in going there. Sometimes, I offer a swimming photo canister as an alternative, and then they sometimes change their home.