Astro Eddy Lifeproof

Hi - we have a query we have 2 granted probates, the first one is our late father his valuation included an investment isa, after his passing we were advised to transfer the investment into my mothers name after his death. On our mothers passing we completed a new probate valuation but only included her part of the isa value minus our fathers portion. We are now trying to draw down on the investment and have been told the value is much higher that that probated for my mother. The sum of both probates is higher than the new total investment value but obviously across two completed probates. How do we amend my mothers probate to include my fathers portion (nil rate) so the portfolio can be drawn down. after spending over two hours on hold we are no further forward today. your help would be appreciated. just to mention both will are discretionery trust wills with a letter of wishes