Chronic medication guess.

Chronic medication should be delivered every 28 days but with Pharmacy Direct it is a guess when they will despatch your medication. My payment dates are as follows to prove they can't deliver to a 28 day schedule. 27/01; 26/02; 18/03; 16/04; 17/05; 26/06. Is it so difficult for Pharmacy Direct to stick to fixed time table? Not a dependable medicine supplier.