when you level up , there is a negative bonus for getting experiance vs low level enemies.
in other words , when you kill a level 10 monster when you are at level 30 or so, you will not gain mutch exp anymore. when the difference is to mutch , you will even get ZERO exp for killing these enemies.
So maybe this is the case in you situation.
In fact every time I level up I get fewer exp points for killing the same monster type. But anyway, I am talking about getting 1/2 exp for killing dark forest creatures and sewer lizards so thia is not the case.
Some people have found that summoned or resurrected creatures give full experience points, but casting Aura of Command on them drops this to half. Others have said they always get half experience.
Definitely I used to get full exp points for summoned or resurrected creatures with Aura of Command on them, but now get 1/2 so I am thinking it might be product of my clvl. I really need to test it more to know for sure. I could go bacck to recent saves before lvling up to 30 and check I suppose.
I remember what my 3rd point was. Sometimes when my demonic aide and I are both beating on the same creature, when it dies we both get full exp points. In other words I receive double the number of exp points I'd get if I killed it on my own. It is hard to tell, but it might be the result of both of us simultaneously delivering a finishing blow. Then later that same demonic aide again gets only 1/2 exp points.
I am going to play a summoner sometime soon. I was going to make a warrior with loads of passives to pass along to my summoned knight, but no one seems to know exactly what passives get transferred by that "Transfer Powers" skill. Death Strike is passive - has anyone ever had a summon perform a death strike?