regularity

Usual culprits are:

Less food in = less waste out
Too little fiber
Too little fat
Too little activity
Not enough water

Seems like you've got the water part covered, so I would look into increasing fiber, fat, or exercise. Other things usually suggested include caffeine, eating on a regular schedule, getting enough sleep, and drugstore products.

Magnesium can work sometimes -- it is a stool stool softener.
Check with the pharmacist about something like phillips milk of magnesium.