Why can't I be summoned in Dark Souls 3?

Solution 1:

Some things to check:

1. Make sure that you are enkindled.
2. Make sure that your firewall is not blocking the appropriate ports. This is likely not going to be the case, since you have stated that both of you can connect to other players online.
3. Check that you have not been soft banned. You can check this by using the Way of the White Circlet (again, this is unlikely due to your ability to connect to other players).
4. Double check you password - passwords are case sensitive. So for example if you are using "DOOM", and she is using "Doom", they will not match up.

Also, just to clarify, your covenant does not have any affect on your connection. Additionally, if you are in a "busy" area (e.g. in a common PvP spot), it might take longer to connect properly.