Does you enlarger light source need time to reach it's working temperature?
I need to have my cathode type on for about half an hour to get it to stabilize.
Even so as it gets warmer the time values change.
So, the time will vary from the first set of prints to the last.
Of course if you have the simple 150/200 watt bulb with condensers you will have avoided the above problem and consequently your problem is elsewhere.
Let us know your results when you get it sorted.