When the drop happens is a funtion of:
-initial soil pH
-initial soil lime content
-fertilizer solution pH
-source water hardness
-re-poptting schedule
-pot size

You can add a little more lime, but I would worry about adding TOO much. Instead, potting-up into only slightly larger pots, once every ~5 weeks, and making sure that your nute solution is not too acidic, will make your life easier.

Contrary to popular belief, it is not stressful to anyone but the grower to re=pot in flower