A quick demonstration showing how firm pressure can help. Don't worry about trowel marks or the finish - just concentrate on obtaining a flat even coat. The rest will become possible as the plaster firms up.
Tip:
If during any of the following stages the plaster is too dry and cannot be 'worked' properly then flicking a little water on the wall with your brush should liven it up a little.
If the whole wall is really dry due to the fact that you may not have fully controlled the suction then sometimes it can be easier to flatten the whole wall, soak it with some water and apply another coat of plaster.