Happiness Meter on the Mix? Kind of confused

Okay, so I got myself a Mix a few days ago for my birthday, and while I am having a blast with it so far, I'm... a bit confused on how the rainbow meter works. From what I've read it's some kind of happiness meter, and you can raise it up by doing various things (from what I've found, playing with toys and taking it to TamaTown raises it up, among other things I can't remember), but... there are times where I can't get it to go above a certain color, and I'm a bit concerned as to what that entails.

For example, yesterday, it would stop going up at the cyan blue square about a third of the way across the meter, and today it's stopped on a greenish color. Neither of those, to me, are even halfway across the meter, and that concerns me.

Should I be worried? What can I do? Am I worrying over nothing? Is this normal??

The happiness meter will only go so high depending on the growth stage (baby, toddler, teen and adult) and if you do the same thing over and over (like feed snacks) the meter will fill up slowly you need to add variety to how you fill it (but really there is no real plus to spending the hours it can take to fill the happy meter 100% it just takes longer for them to make the pouty face)