Initially only thing I can think of is the level you are compared to friends level ? and then the level of targets you are killing
Very odd
The only thing you can do is trigger a file verification per this link and see if it has any affect
Can you check this other thread here and see if what suggested there resolves your issue or is the cause.
What does it show in the chat log of your general chat or the other tab? Hover your mouse over the General or the Other tab, right click and then left click on "Chat Settings". The last section "System Channels" the very first option "Combat Information". Make sure that box is checked / orange square and then click ok and try again to see if any xp is reported for kills for either group or solo kills.
Wait if you help a guy in his quest, do both get xp or only the guy doing his quest? Because the quest the guy wants you helping him is in HIS mission log but not yours. It could be this case.
