Re: "Don't use strength!"
You are right to assume you have reached a 'plateau'. One of probably many. The good thing is they probably tell you this because they think you are ready to start a new development in your aikido. It's really a difficult transition to add a new aspect to your aikido and to forget what was once 'the truth' to substitute with a broader perspective.
The bad thing is that this will continue at least for some years ahead - probably for the rest of your time as long as you practice aikido (or any other physical art).
The good thing is that the bumps tends to become slightly smaller and more apart with time, and that every time you reach such a plateau you will eventually find yourself on the other side some day. I had a few of those periods before reaching shodan and some again afterwards. I'm even having them now where I am teaching in my own dojo and have started a new way of looking upon the development of my aikido.
Just don't worry too much. Try to enjoy the exploration of new facets within your art and have faith in the thought that one day such exploring will get you to a higher understanding and a new level. I for one believe it is well worth it - and that it contains one of the very valuable lessons one can gain from Aikido.. so just push on and adapt whatever you can from the instruction you get - leave the rest for some other time. It may one day suddenly make sense even though it might sound strange and unbelivable right now.
Hang in there - and have fun.