What you're describing generally means that not enough energy is being applied to the moving parts, relative to the resistance.
Essentially, the BCG isn't moving backwards with enough force to overcome friction and the weight of the buffer and recoil spring, to then get flung forward with enough energy to strip a round off the mag and then fully chamber it. So, you need to either add energy, or reduce resistance.
The only ways to really add energy to a DI AR is to either increase the amount of gas going through the gas tube and into the BCG (adjustable gas block), or to use more powerful ammo. Your problem may also go away by adding a suppressor - the backpressure they generally cause means quite a bit more PSI being applied to the bolt face after it unlocks, increasing its velocity backward.
To reduce resistance, you can either drop to a lower weight buffer, or a lower powered recoil spring. I'd go with lighter buffer first, as you want the energy that spring absorbs to be used in flinging the BCG forward again. A lighter spring will generally mean less energy being applied on the forward stroke. Also, be sure to be using a high quality, very slick gun lubricant to minimize energy loss from friction - something on the thicker side, not something thin like CLP or Rem Oil.