After changes have been made in the IHT205 or IHT400 by Probate Plus form filling, the form isn't immediately aware that you have made a change (as it's expecting you to manually enter information into the form).
With Acrobat (PDF) forms, to force the form to recalculate, simply make a small change to the form - such as ticking / unticking a tick box.
With Oyez forms such as the IHT400(all), moving between pages or fields manually usually triggers the form recalculation macros.