Feeding problem on XD-.40 sub-compact...is this normal?

I've got a minor feeding problem on my sub-compact .40. By minor I mean four incomplete feeds out of 240 rounds. So far I've shot 200 rds. of cheap Winchester flat-nose FMJ (you know, the white boxes at Wal-Mart) and 40 rds. of nice Winchester personal protection stuff. I've had the same problem with both types of ammo: the previous round is extracted and ejected fine, but when the slide picks up a new round it only gets about half-way into the chamber and fails to go into full-forward battery.

This is easily solved by a tap forward on the back of the slide and it falls right into place, ready to fire. This has happened once with the personal protection ammo and three times with the cheaper, heavier ammo.

I suspect this is the result of the sub-compact's two-spring guide rod [correct me if I'm wrong, but I think the SC is the only XD with a 2 spring rod]. The primary spring is rather weak, maybe @7lbs and that takes over from the secondary spring about when the cartridge is half-way through the chamber. That's what makes me think this may be an issue only for sub-compacts. Perhaps I'm over-oiling, or there's some quick fix, but I'd really like to get better than 97-98% reliability out of this pistol. I hope I'm not asking too much. Other than that it really is a fine pistol. It's my first self-defense gun and I kind of want it to be the last.

For all you guide rod gurus out there, is there a nice fat 22# spring for the sub-compacts? What's the rationale for the two-spring set-up in the shorter barrels anyway?