you know, i had to find this out myself some thime ago... but anyway.. wi you play real battlenet games, you will notice that heroes cant kill neutrals (creeps) to lvl past lvl 5, its a balance thing
at lvl 1 a hero gets 100% exp for killing a creep
(im not sure exactly how much) but a lvl 2 hero gets 20 % less
ex : a creep dies and gives 100 exp, a lvl hero will get 100 exp
the same creep dies and a lvl 2 hero only gets 80 exp
at lvl 4 a hero gets 20 exp only, so for 100 exp he needs 5 creeps
now to solve your problem, go to gameplay constants, find a value
hero Exp reduction 20 50 80
(or something similar, im austrian and have a different WE)