Is it possible you have a window active for something else when you press F5? JTAlert has to be the active window for F5 to work. I had the same problem until I realized I was making a windows mistake.
For example, I would select a call sign in JTA, then go to WSJT-X to adjust power. At that point WSJT-X's window becomes active and JTA's F5 won't work without reactivating the window.
I'm assuming you are using a PC with Windows.
Once I click on the JTA window, if it's not already active, I find it's easier just to continue with a few more mouse clicks than to use F5.
I like the text feature. I'm enjoying exchanging brief messages after making a contact.