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 Laserforms IHT400(+) suite or Oyez Legal Forms such as the IHT400(all), moving between pages or fields manually usually triggers the form recalculation macros.
Some forms have options to enable/disable calculations and various form behaviours. Make sure that you've selected the correct options to enable calculation.
Finally, make sure your vendors form library is up to date as you may be operating an old version of the form.