The fact is that Northern is not short of electric units. The root cause is the enormous variety of units that Northern operate, and the headaches that come with that -including increased training and less inter-operability.
A few years ago, the answer could have been the replacement of the Northern 321 (4 units) and 322 (4 units) fleets with some 319s. Replacing 2 classes with 1, and the added bonus of units transferring from either side of the Pennines if there is a temporary shortage, such as now. But impeding 331s mean its not currently worthwhile to do that, so passengers will have to live with short forms and cancellations.