I bet you have the same FR1 water filter housing as I do. I actually talked to a guy from WaterPur. He claims the problems people are having with these filter housings (leaking and un-screwing) is caused by thinking they found a replacement filter at one of the Big Box stores. If the filter is just a shade longer the tabs won't catch and lock. He also said to use a silicone plumbers grease on the o-ring. Shop Danco Silicone Grease at Lowes.com
(NOT TO BE CONFUSED WITH SILICONE CAULKING)
Also, make sure the red arrow on the clear housing lines up the with pointer looking arrow on the top part of the white housing while screwing it on. It just happens that the red arrow on my the clear housing lines up with the pointer that's on the back of the housing when properly tightened. If that's your case, don't try to make another full revolution to get the red arrow on the front side. There are pointers on the front and back.
If you keep your FR1 filter system use only filters for the FR1. They are pricey compared to other whole house filters out there, but don't substitute and take a chance.