Is there a chance that something is summoning additional mobs? Fairly new to the game myself, but in many games you don't usually gain full xp for something that has been summoned as a result of another mob. I have noticed that certain enemies have this ability and have not paid much attention as to whether I gain full xp from their spawn or not.

I ran into a similar situation with the Wood Elf and this scenario you had posted came to mind, so I hope the information I am about to share will help others who come seeking a solution to this very problem. Sacred has a system set in place that provides diminishing returns on experience based upon the level of the one who lands the killing blow. In the case of Wood Elf and Vampiress (both possess summoning abilities) it is a well documented case that if these summons outlevel their kill too much, then the party is only awarded 1xp. It is best practice to keep all summon skills as close to your own level as possible, to prevent this from happening.