i have encountered this problem myself. the only answer i could come up with is which general are you selecting for the enemy AI? if its a specific general and not just USA,GLA or China... there's your problem.
I know its a bit annoying coz some of the specific generals are more of a challenge, but nothings more annoying than building a wicked base, then a group of attacking things (tanks,infantry,aircraft,etc) then finding out that the enemy AI takes 2 seconds to kill.
Hope this helps, as i joined up just to let you know this;)
