It is entirely natural and expected that your performance in an assessment run may not be improved upon from your previous time. It is important to appreciate that this is nothing to be concerned about, and does not mean that your training is not beneficial. Excluding reasons extraneous to your plan, there can still be many reasons. The simplest training related consideration is that your training is targeting your goal, not your assessment. Consequently, improving your 6 minute time ‘today’ may not be beneficial for your marathon race in ‘x’ weeks time. Furthermore, your assessment performance has to be considered in context of your previous training. On the day of the previous assessment you may have been in a far more rested state and consequently able to perform at your best.