@acer I think I now know why. What I first did was to convert the bold a first to an atomic variable and then I type := value to assign. But that didn't work. I then tried typing everything out and finally converting a to an atomic variable, and that worked.
Having assigned successfully several such variables, I tried adding some of them together. Likewise, I first tried converting each one to an atomic variable while I typed the expression, but that did not work and some of the variables were used as the non-bold ones. I then tried finishing the expression first and then click on each bold variable to convert it to an atomic one. That worked.