Ensure the plaster is not too thick – the peak test with the whisk will ensure correct consistency. When the whisk is pulled out of the plaster it should make a peak of around 1 inch. If it is any higher then add a little water. If the plaster is too thick whilst it is easier to handle on your hawk and load the trowel - it can be harder to keep it flat on the wall. The first priority must be too keep the plaster flat – otherwise when the plaster begins to firm up the trowel will not be sitting flat on the wall and a smooth even finish will be difficult to achieve.
Slow down and focus upon what your trowel is doing by looking at the difference it has made to the plaster. Read the wall to see what needs improving
Remind yourself of the angle of the trowel and pressure needed and very slowly try and rectify any problems - but remind yourself of the objectives for the stage that you're on - don't try and jump ahead. Once you've got the right plastering technique and the ability to control your trowel accordingly very soon muscle memory will take over and your speed will increase.