Entry Door Leaking

We're building a new house and installed the exterior entry doors a few months ago. When the rainy weather started last month, we started noticing water leaking around the bottom of the door (Between the bottom of the door sill and the subfloor).
The door is a "Reliabilt" prehung steel entry door we purchased from Lowes. The weatherstripping all seems to be in good shape and contacts the door nicely. The adjustable threshold on the sill is set so it contacts the weatherstripping on the bottom of the door.
When I installed the door, I ran three thick beads of caulking on the subfloor, with extra at the ends, and ran a little about 6" up the stud on on each side of the door as well. Once the door was plumbed and fastened, I caulked all around the brickmoulding, both where it meets the wall, and where it meets the door jamb itself. I also caulked around the metal sill where it meets the wood jambs, up against the adjustable sill, and along a "groove" in the door sill itself.
The door is in a gable end wall, and there is no real roof overhang, other than the 6" overhang at the end of the roof itself.
Despite all these measures, the door continues to leak. I'm stumped. I can't figure out where the water is coming in.
I had a similar problem on our mobile home several years back and couldn't track that down either. My solution then was to install a storm door on the outside, but I would prefer not to take that approach with this door.
Am I missing something?
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