Not 100% sure if this would be the source of your issue, but First Aid Kits and Medikits are mutually exclusive. If a soldier has a First Aid Kit equipped then that soldier cannot equip a Medikit. Unequipping the First Aid Kit will allow equipping of a Medikit. This is less than ideal but was pretty much the only way to get the First Aid Kit working. I haven't figured out a way to actually make perks trigger abilities (as in actively used ability icons). So both First Aid Kits and Medikits function pretty similarly, and trigger the same ability. The amount of healing/immunity to poison is determined by which item the soldier has. The logic if a soldier had both was impossible to resolve. That was exactly the problem. I had to unequip the 1st aid kit completely. I was trying to replace the 1st aid kit with the medikit. Thank you.