You might be running into a thread problem. There is some taiwanese supplier who seems to find their way onto many boats, and they use a non-standard thread pitch. Many deck fittings are 1-1/2" pipe size with an 11.5 TPI thread cut. This matches standard pipe threads for that size pipe, but it's a straight cut not a tapered cut. The mystery supplier instead uses 14 TPI. This is what I have on my boat, and it was a real bitch to figure it out so I could make a pumpout fitting with a banjo coupling. Nobody makes an off the shelf fitting for it.
One suggestion would be to try MMI Marine Replacement Parts for Taiwan Boats | MMIMarine.com
They have quite a few of the asian parts used in many boats. Check the web site, but you will likely need to call them. They are very helpful.