Sometimes it takes a couple of tries to get a friendship to stick. As others have said, you both have to accept the friendship again when returning home, unless one of you is on your home grid, then only the one returning home gets the pop-up.
I friended a bunch of people on HG Safari (a tour group that meets every Wednesday, in season, to visit other grids/regions led by Thirza Ember) one time, and I had to relog several times for the confirmation to pop up for everyone, as only two or three would pop-up at a time. There have been times when even accepting it again from home, it didn't stick, so we had to go through the process again. Be persistent, and eventually your friendships will stick.
Another thing I have noticed is that sometimes, even though someone is online, I can't see that they are online, and vice versa. I have an avatar on one grid that can't see my avatar online on my grid, and they can't see me either, but I can send a message between them, and it comes through. We can't teleport each other. So if I need them to be together, I have to use a workaround.
Sometimes people will show online, but they are not. I think this can happen if they log off from a grid other than where they have their account, like if they crash. But not always. I have one friend who looks like she is online all the time, and she isn't. The weird thing is, we both have our grids on the same server. Our communication is hit or miss on my part. She can see me online and doesn't get the same phenomenon of my ghost, so she usually is the one to message me.
Open Sim is fickle and finicky, so persistence is key. I have friends from many different grids, but communication can still be a challenge with some of them. I hope this helps. Good luck!
like(1)