I never had issues with mages in DS1 reaching levels at the same speed or levels equal to my rangers or melee characters. One of the reasons you might have trouble with mages when working with rangers is that rangers tend to have a greater sight line (can see monsters sooner than mages) and bows tend to out range spells. So often the monsters and bad guys are dead prior to the mages using their spell. Suggest when you hire a new mage switch your ranged characters to magic spells and make the new hire leader of the party. That should help increase their level because they will be the first to engage and use their spells first. You could also use a mod like Kane's Rings and give the rings only to mages so they will increase levels faster than the rangers and fighters.
I never had issues with mages in DS1 reaching levels at the same speed or levels equal to my rangers or melee characters. One of the reasons you might have trouble with mages when working with rangers is that rangers tend to have a greater sight line (can see monsters sooner than mages) and bows tend to out range spells. So often the monsters and bad guys are dead prior to the mages using their spell. Suggest when you hire a new mage switch your ranged characters to magic spells and make the new hire leader of the party. That should help increase their level because they will be the first to engage and use their spells first. You could also use a mod like Kane's Rings and give the rings only to mages so they will increase levels faster than the rangers and fighters.