Very old house !

Im applying a finishing coat but because it’s different thicknesses it’s drying at different times and it’s making smoothing out very difficult. Some areas are drier than others, as in Im leaving trowel lines in some area. ?? Any advice is welcome !!

Answer
You have there perfectly described the reason for a backing coat.

So it sounds like you need to flatten the wall with the thinnest of backing coats - so that your finish coat can be applied at the same thickness.

Paul