Yes voodooqueen. You do make a good point.
I think the Andalite problem could have been one thing: politics. You know how they are. Delay of decision, argument, moral struggles, economics, political correctness, finance, everything. the whole lot of it.
and like JFalcon, I had always thought the Yeerk Empire to be rebels who took off from the planet as soon as they saw an opportunity.
However, JFalcon, you hinted at an interesting possibility. Perhaps, the reason why, and this is purely dreamily hypothetical, the Yeerk Empire was broken apart from within by a mix of Yeerk Peace Movement groups, Taxxon rebellion, propoganda, and Andalite spies. This could explain a few things.
1. the lethargic response of the Andalites. They weren't fighting the Yeerks head on. They were planning and acting sabotage from within.
2. This could also explain the arrogance, pride, and stand-offish-ness of the Andalite and especially Andalite military. They have secrets to protect.
3. The behind the scenes damage would be unknown to the Animorphs and thus, a legitimate excuse for why nothing was mentioned in the series.