I would agree with those above that it is a fatigue issue. You completed these plans last time but with more recovery due to skipping workouts. Also, you are in your fifth week of continuous loading so your fatigue should be at it’s highest. I don’t know your age, training history, lifestyle, etc. But maybe the issue isn’t that you can’t do the MV plan but that 5 weeks of loading is just too much. I would just skip ahead to the rest week and drop that fatigue. Before you drop down to a LV plan I would consider this first. Build will be more fatiguing than Base but it is also a 3 on-1 off pattern as opposed to the 5 on-1 off pattern. So even though every week is more fatiguing you may get on better with the more frequent rest weeks. So LV might be the answer. But you also might want to consider an alternate work-rest patterns for base the next time you go through it.
Disclaimer: I’m not a coach but just wanted to put another option out there to consider.