If I had to guess, I'd say that they didn't use a little silicon plumber's grease on the o-rings, and damaged them when installing the cartridge or did not get it fully seated. The o-rings on the back fit into the holes at the back of the universal rough-in, and the one around the body seals the cartridge into the rough-in. You will have trouble seating the thing by hand-tightening the threaded ring...you really need to seat it, then tighten the ring only as a retaining ring, not something to seat the o-rings in the first place. It would be fairly easy to check - turn the water off, take the trim off (handle, you can leave the eschuteon), then remove the ring that holds the cartridge in. Take a look at it...you should not be able to see the o-ring - it should be inside the rough-in. If you can, then slide the cartridge out, put a thin coat of silicon plumber's grease on both the big one on the outside of the cartridge, and the two smaller ones on the back of the cartrudge, then pop it back in place. Then, put the ring on hand tight. It will leak if the o-rings aren't seated or damaged. Trying to jam them in without some grease could nick them as well. It takes a LOT of force to pop one in witout any grease, and many people may not be able to do it and try to use the ring to seat them (probably won't work, especially if you rely on hand-tighten as the instructions say). If you grease them with a light coat, it still takes some pressure, but is doable by an average person.