I am running a campaign in Roll20, and I have some players that will very often miss sessions without any warning, or just show up really late into the campaign. Because of their absences, I have had to cancel the last three sessions. They're not new to how RPGs are supposed to work: one is really experienced, the other is has played with three other groups before, so both have some games under their belt.

I have asked them numerous times why they never let me know when they will be late or not there, and have never gotten a response. I really don't want to kick them, what should I do?