The point is, we really do have strong days and days when our bodies are tired and it;s not always apparent why, In my case, I just forgot I’d done a workout. (I felt it the next morning though!) But it can be anything from your body still recovering and building strength from your last workout to a night of bad sleep. From a mile high view, you’ll always be improving, you just have to breathe, remember you aren’t backsliding, and give yourself a break.
It took me 3 months to be able to do a pull up. That’s 3 month of hanging on a bar, willing my arms to move, and failing. I slowly built up strength doing assisted pullups but WOW was that a frustrating three months! Then my ability increased exponentially. At one point I could do 8 in a row from full extension. Be patient, be kind, and trust. Your body will gradually build strength and flexibility without you having to overexert or be better every time you practice.
Source: Rx Primal Blog