Is Minas Tirith the last Gondor`s city? If it is, then there is a chance that Gondor is being destroyed, and when a faction is getting destroyed, then a script should kick in which spawns multiple stacks of Gondor units as to help the weakened faction survive. For some users this script takes a long time to kick in due to their processor "processing" the script.
How long have you waited for the turn to end?
Also, adding this red line to your TATW.CFG text file helps ( file is located in your M2TW/mods/Third_Age_3 folder ) :
Save and Exit. Then end your turn and wait for 10 minutes.. Read a book, or make a cup of coffee or tea. 
If it doesn`t work, upload your savegame file so someone could test it.