Interesting discourse! I think whta everyone needs to remember, is that we are not a single-faceted being - each and every one of us - Dom AND Sub are multifaceted- each of us affected by the ebb and flow of our days and the various demands and stressors.
I do NOT think you should be hard on yourself for "not feeling like a sub" sometimes - what I DO hope is that you have a Dom/Master who understands that each of us has stress (usually the main culprit) in our lives that makes it difficult sometimes to get into the spot where He/She wants us at that moment - it is NOT just about being the "perfect" submissive - it is also having a caring, perceptive and nurturing Dom/Master.
I live in a a very long-term 24/7 D/s relationship and yes, there are days where I'm not in the mood and he is - what I have found is there is no simple answer to each of the occasions - there are times when he will very successfully get me in the mood by an exellent spanking - I still find myself grinning that the old adage "she needs a good spanking" really DOES work for me - it is a super effective way to "de-stress" and relax and D. learned this a long time ago ...
I know this is long-winded but I think it is important to differentiate between the fantasy of being the perfect "sub" (or for that matter, the perfect "Dom") and the reality of every day life - it doesn't mean you can't find ways around it and work together but the important thing is to COMMUNICATE - D. smacked my ass the other day and I snarled at him (an unusual thing for me as I ADORE my ass smacked usually) - he initially reacted angrily then sat me down and asked me what was wrong ... I had just worked 60 hours in 5 days ... I ended up bawling about how tired I was ...he ended up tucking me into bed, bringing me a hot cup of tea and then turned out the light and told me to go to sleep ... the next night, I MORE than made up for it ... grateful for his understanding and nurturing.