Summoning Failed

I'm playing on PC, and my friend and I started trying to get co-op going this morning but we keep getting summoning failed. The thing that's confusing to me is that I've been summoned into random players game plenty of times and even been invaded, but when my friend or I try to summon each other or even randoms we get summoning failed. The steam forums don't seem to have any info on this problem, so I was hoping someone here might know what's up.