I bought a safariland 6378 for my Sig P229 SAS recently. I love how the paddle fits my hip and the mechanism for releasing the handgun. My only issue with the holster is its really tight still. I put my Sig in the holster and left it holstered for a couple days to stretch it a bit and it has helped. I still have trouble holstering the gun and getting the lock to engage. If I forcefully push the gun in it'll latch in but if I just place it in it will not latch. In Safariland's videos on their website their demo holsters look much looser. What adjustments can I make to fix this?

You may also want to play with the tension adjustment screw, on the side of the holster. It may be cranked down tight.

And yeah, the ALS holsters need a short break in to smooth out the lining. They get better with age.

You can also stick your gun inside a thin sock or plastic bag and place it in the holster for a couple days.

Some folks have advocated spraying the inside of the holster (the suede lining) with silicone spray. I do the opposite: I use a silicone gun cloth to wipe my pistols down at least once a week. Over time, that silicone will transfer to the suede lining and make for a slick draw. YMMV, just a couple more thoughts.