Supposedly, All Might injuries are one of the reasons he can't last and fight anymore like he did in the past, before the battle with All For One. Why Recovery Girl never cured All Might injuries? Midoriya many times completely broke their arms and legs, but she was able to completely cure him. Why this doesnt work for All Might?
Her power only goes so far a few spoilers are ahead so if you haven’t finished season 2 then move along
During the UA quirk games Deku breaks his arms and fingers multiple times in the fight with toderoki. Recovery girl explains that she can’t repair everything and because of Dekus lack of restraint and the damage he caused he now has a perminint scar on his hand. If recovery girl couldn’t repair him enough to not leave a scar then I doubt she would be able to deal with the magnatitute that is almights wound. She probably healed to the best of her ability so he wouldnt die ( as his wound has now become I giant scar) she can heal and it’s a miracle but she isn’t a god
As Recovery Girl explained her quirk only accelerates a person's natural healing rate with a proportional cost to their own energy.
So since Deku's bones could heal over time her quirk could make them heal faster, but you wouldn't expect a person to be able heal having a good few major internal organs crushed over any length of time so Recovery Girl can't repair All Might's injuries.
Recovery girl can only boost healing factor of one's body. All Might lost some part of lungs, stomach etc during fight with All for One.