Can Balloon Valve Operation Be Done For People Who Are Overweight?

hi my nan is currently in hospital and she has been there for over 3 weeks she needs a balloon valve operaton but the surgons will n ot operate saying it is due to her weight.
she has been told twicw it will be done and on the day they cancel it.
i just want some help to uderstand why noone will operate as to me they have my nans life in there hands and not doing anything.
any information will be greatfully recieved.