Why the change in clutches?

I bought a proven Ocellaris pair this summer. After 6 weeks they laid there first clutch. they had 2 more clutches around 13 days apart before I was able to get the fry past 3 days.
The 4th clutch was large but it appeared that only part of the eggs developing. I hatched out this clutch and the fry are looking healthy.
The last 2 clutches have been of good size but the time has been 28 days for the last and most of this clutch is gone on the second morning.
Why has the time between clutches changed?
Why are only part of the eggs developing?
What can I do to make it better?
Thanks for all the help.

You need to ascertain the reason why they have longer periods between laying. Diaries always help. Could be nutrition, temperature or a whole host of other things. Or it could just be the cycle of the female. There are no hard and fast rules with spawning, but nutrition, a good varied diet and stable parameters are key.

I have been keeping a diary since I started this. Food has not changed. I am feeding Mysis, Rods fish only, some table shrimp,cyclopeeze, spectrum and marine blend form Brine shrimp direct.
 I have been off work for 5 of the last 7 weeks so all the fish have been fed more than in the past.
Lighting has changed a little. Daylight savings time moved the clock and there light by an hour. Being off work has made the time the lights on be different than in the past.