Maybe check your temps? I'm not sure if it's something you need to really worry about just yet...
I have one gecko that could star in a metamucil commercial-- she poops at the same time everyday. My other is a little off; I think the longest she's gone has been four or five days also. They're exactly the same age and kept in the same vivarium, so I imagine it has to do with the fact that one is a pig that almost never skips a meal, and the other is easily put off food by stress and generally just doesn't eat as much anyway. Also, the piggy one is clearly the dominant one, and she gets the warmest spots in the viv. The other has access to the ideal basking range, but she doesn't lay around in it day in and day out like the other. Like you said, the one that has some irregularity is still eating, growing, and generally healthy, so I've decided to keep an eye on it but not to worry...