Thanks Mike, I never reuse developer, to me that is just asking for trouble. So I used it one shot.
The BTZS tube cap holds about 80 ml, which should be enough for 1 4x5 sheet, which is the size I use for testing. If the problem was exhaustion I should see a difference between the 7 min and the 14 minute, yet, they are the same....

The only thing I can think of is the developer getting some extra humpf as it sits, so I will do as you say and mix immediately before use...we will see what happens.