Hmmm. That sounds strange doesn't it. There's only one thing that comes to mind which is - are you being quite forceful about seating it? Sometimes if you just gently push this type of shaft into place it all looks very nice and everything seems to be properly seated but the ring will only expand out into the dif gear if the shaft is really rammed home with some quite considerable force. Just pushing the shaft into place is sometimes not sufficient to do this. The inner tripod joint is a "plunge" joint so the drive shaft can move linearly in and out of it. You can use this to your advantage by plunging the outer CV and shaft towards the gearbox in effect using it like a slide hammer. Obviously you don't want to go totally ape with this or damage to the tripod needles may result but it's a robust construction and doing this, using "sensible" force has always worked for me up 'till now without ever damaging anything.
Alternatively, if you're not confident in doing this you can release the inner joint rubber boot enough to allow you to place something like a piece of hardwood against the rim of the joint so you can then clout the bit of wood and drive the joint inwards towards the gearbox. Unfortunately this usually results in the boot clip being destroyed when you remove it but new clips are available and actually I've had great success by winding galvanized iron wire twice round the boot and twisting it 'till it's tight on the boot. You need to shift the boot otherwise it'll be cut by the, relatively sharp edge of the joint when you hit the bit of wood. Don't hit the joint directly with a hammer because it's hardened so brittle and you may get bits chipping off.
From what you are describing I think it's very likely that the shaft with the circlip on just isn't quite getting pushed far enough "home" for the clip to fully spring out. it's probably only missing it by a mere "smidgeon" and a good "tap" will seat it. - Then again, being as I'm not infallible and don't have a crystal ball, maybe not?
PS Mostly the problems I've had with this sort of ring is either that it breaks (often when being removed) so, no longer being a circle, it can't exert radial force on the diff gear so it won't hold - or with it jamming so you have great difficulty getting the joint to disengage from the box. I can't really remember having to replace one because it had become "weak". - Although maybe the manufacturers are concerned about this being a possibility because you'll often find a new "snap" ring included in repair kits.
