That's right, always blame the crews. I know for a fact that my TOC requires very high fleet availability from their units, with very careful planning required for units that need maintenance and the like. The units are currently being fitted with GSM-R as they go through the main depot - this also requires careful planning. Let's say you each four car unit is worth a million quid a carriage, so a four car is worth 4 million notes, and you've got 12 of them sat around doing nothing. That's fifty million quid's worth of assets doing nothing, all depreciating in value and earning nothing while they're sat in your carriage sidings. For that reason, TOC's have very little spare stock available outside of the peak times!!
Some weeks ago, one of the units booked for the Bedford line failed, and the local on-call fitter could not make it better. Due to the availability required from the units, a service up in Birmingham had to be short-formed so they could pinch the unit for the Marston Vale line.