Why does FSDT disappear after 5 minutes?

[paulyg123]

I have al the FSDT airports installed and all work great.  I see all of a sudden, KMEM will load perfectly and then all the gates and textures simply disappear in about 3-5 minutes.  Usually when I am still at the gate of starting to taxi. 

[Captain Kevin]

Installed, but activated or not. Trial version will disappear after a few minutes.

[paulyg123]

OK, they all were activated and working.  So here is what I did:

Opened up P3D (the trick is to remember the ADD ON Manager resides in P3D)
Went to Addon menu at top
went to KMEM and saw it was not registered.
hit the register button and put in my license
It accepted it and now it works. 

I needed to do it for 3 other airports KLAX and KIAD but not for KJFK and KCLT.
For some reason, several airports just dropped the reg. key and de-activated!  Why? I haven't a clue.  But they all seem stable and working now.