Is anyone else having trouble healing the dwellers? Mine will not heal and needing it bad

Is there red in their health bar? If there is, that red represents radiation damage, which reduces their maximum health. That is to say, if half the health bar is red you can only heal the non-red half.

They take radiation damage whenever your water drops into the red. RadAway will heal that damage, but if your water gauge is still red, they will continue to take radiation damage.

Gathering enough water to turn the water gauge green, will stop the radiation damage from increasing and over time will slowly heal the radiation damage.