They've always had good spells and bad spells, availability wise with ScotRail. I've had whole diagrams that were booked HSTs and every train has been a unit.
That said, the past month or so I've not had a unit covering for an HST once. I've maybe just been lucky, but I do have a sense that they've finally recognised that they need to improve the availability and that they put a plan together to do this. Four OOS power cars are currently off getting returned to traffic, at Loughborough / Kilmarnock, and there's a programme of modifications for this summer including upgrading the WSP system. To give you a sense of how big a deal that is, 43128 has never worked a revenue earning service since coming to ScotRail, and now its getting returned to service after years of being used to provide spares for the rest of the fleet. Two ex LNER power cars have been obtained to spares donors to get 128 (and 169) back into service.
I suspect the reason for this is twofold - they looked at options for replacing the HSTs and found nothing suitable (meridian sets need a refurb, ex TPE mk5 sets are too small a fleet to cover the work). On top of this, Leven opens in June and that stretches an already intensely used DMU fleet about as far as it can possibly go.
I think on the back of these realities, they've decided to spend a few quid and improve the availability of the HST fleet.