"Why did they arrive late?"

Translation: Kial ili alvenis malfrue?

[Miguel.la.Liebre]

Why 'venis' isn't accepted? Veni in its pure form means 'to come/arrive to a certain place', then should I raport it?

[mizinamo]

I disagree; I would say that the basic meaning of veni is "come", not "arrive" -- that is usually alveni.