Bought a triton shower the inlet is made of hard plastic ,instructions say to use a 15mm compression joint but when I put it on hand tight the compression joint was not gripping, could not tighten any more as there was no thread left, what can I do?? Thanks

nstructions say to use a 15mm compression joint 15mm= 5/8ths.
Did the instructions say if the 15mm was ID or OD? Makes a BIG difference. Tom