That is really negative. We all have times when we drag ourselves to Aiki. If you are saying you have never felt that then you have not been training long enough.
You say you have never had a class where you have not learnt anything - well, I find that surprising, but if that is true, you are the luckiest Aikidoka alive, and probably the only one.
Don't be afraid of plattos. Progression in Aikido is not a slope but more like a series of steps. Get better - stay the same - get better - etc...
Also, don't worry about getting worse either - it usually happens before a big improvement!!!