Yeah it may not be seated correctly or in a clean and silicone greased hole. I have an old house and my brother who I help does to and what I've noticed is when I replace cartridges for old shower or tub valves, I have to take the time to clean out where the cartridge goes back in and I use extra silicone grease to make sure the o-rings slide in without any burr damage.
And Twowaxhack is so correctly about using OEM cartridges. When I first bought my old house, I tried grabbing an after market cartridge from Ace and it leaked right off the bat. Ever since I purchase OEM catridges, and it's the correct one, and I take the time to gently clean the opening and use extra silicone grease.... I've never had a leak ever since and I've replaced like 7 in a row like that between my house, my brother's two houses, and my parents condo and their house. (I like to help with plumbing projects and we live in the same area).