Why does he only piddle on the puppy pads?

I'm sorry for all the questions but this is the only place i've gotten good advice on my Chihuahua and his actions.

Now here is the new problem he just made.....lol. :?

On his puppy pads, the bathroom ones, he only piddles and doesn't poop there.

Why?

I mean I literally have established a routine and "code word" for the bathroom with him....all I do is ask if he has to go potty and if he does he whines when the word potty is said. Heck even my husband knows the routine and follows it....lol.

WHen I catch him squating on the floor next to the pad I say no and show him the pad....then and only THEN he does everything on the pads.

But he doesn't seem to poop on them....and yes they are changed out when they need to be.

Any suggestions on what to do?

I eventually want to get him started going outside, for when we have a house later on, but the vet said to hold off on that till she sees him this saturday and gives him his next shot.

I'm at work now and seem to be on a different time schedule then most of you so i'll keep checking back when I get a chance.

All the information I'm getting from you all works with helping to figure out things or how to tweak it actually to fit my little raz.