Campaign budget stopped being spent

Adwords ninja needed! My client does the Adwords itself. In mid-September '20 campaign "died" and only a small amount of the budget assigned to this campaign has been used. I reviewed the account a bit and I realized that the bid strategy for this campaign has been set to ROAS 1000% (Oct '19). But the campaign died instantly almost 1 year later, ads stopped to show and the budget stopped to be used.

After this, my client changed the bid strategy to target CPA, but sill the budget is being used in 40% only.

Any ideas about what happened and how to fix?

Unless adwords have enough data, you need to use Maximize clicks option. And if you are using target CPA, then in order to utilize the budget, dont set maximum CPA cost. Adwords will optimize your budget to get max conversions at lowest possible cost.