WhatsApp loses connection with only one contact

Hi,
I have been using WhatsApp for a long time but lately I am having problems with one specific contact.
Sometimes I am talking with this contact and suddenly it stops, the last seen information also disappears and any message that I send get the one check mark, so it stays in the server and do not reach the other recipient. When eventually the contact comes back any message sitting in the server never is sent.
Nobody loses internet connection and all other contacts work just fine in this same period.
The same problem occurs on her side at the same time. I disappear to her.
None of us block, it just happen during conversations or sometimes we go into WhatsApp and the contact is blank it out with only the name available.
We tried cleaning the app cache, force stop, restart the phone, recreate the contact and not solves the problem. Eventually it comes back, sometimes stay on for a few hours or a couple of days, then stops. Sometimes it stays off for a couple of hours and come back, sometimes it take days to come back.
Again it is only with one specific contact, all others works just fine. This problem starts to happen about 2 or 3 months ago and it is aggravating.
The big problem is the one I need the most and vice versa.
We both use android.
Please, we need help.
Thank you

Thank you Scott, but everything is enabled. As I said, it works sometimes and suddenly disappear. At the same time it works with all others so no internet connection is lost. I was thinking that could be something related to the time clock in her phone that due to some errors could block international contacts automatically.
Today, after I get some another message from a different contact, her contact was automatically enabled back on.