stabilized symmetric power supply from battery
Did you happen to use the "heavy duty" 9 volt battery? They have an amazing capacity of only around 50 mAh!  So if you put any real load on them, their voltage will drop quickly and they will likely heat up as well due to high internal resistance which is tripled when put 3 in series.  Take away the load and the voltage will go back up some.

The same will happen to a lesser extent with alkaline 9 volt which have a capacity of around 300-400 mAh.

You would be better off with lithium batteries, but if you want to keep it simple, consider AA, or C, or D alkaline in series.