There would be an inventory of tunnel liners no matter what - they are not shipped just-in-time, but rather as they are made. They are stored on site as there can be peaks and troughs in the production flow, and the last thing anyone wants is to hold up the TBM because there isn't enough liners to fit.
If the TBM wasn't progressing to schedule, what there would be a lack of, however, is spoil.
One other thing to consider is that TBMs are not turned on to 100% immediately upon starting. The first several weeks or months of the drive will be done more slowly, as they break in the various components and test the different systems. Only once they are confident with it will they then begin operating at the full production rate. That's why that "15m/day" number that gets thrown about frequently is also cited as an average - once things are up and running smoothly, it is possible to double the production of that average number.
Dan