I am thinking it is very hard to give meaningful advice here because there are just SO many variables it becomes a very individualized thing:
Leg strength
Pulmonary function
Cardio Function
Vascular Function
Metabolic Function
Weight of the equipment
Weight of the rider
Equipment efficiency
Gearing
Tire size
Tire tread
Tire TPI
Tire inflation pressure
Type of hill (short & steep vs long & gradual)
Type of turf (Paved, limestone, hard pack, soft, muddy, snowy, etc...)
... Even emotional factors enter into it: Confidence, Aggressiveness, etc...
So, to the OP: What do YOU think the problem(s) is/are?