Driving dimensions / variables

Hi all.
I have not used SE 2D for a while. I am starting over now, with version 20.
Smoother interface.
Here's my problem:
On a new doc, I draw a simple line which I dimension with the "Smart Dimension"
tool. This creates a driving dimension Vxxx, which I can see in the variable table.
If I create a variable in the variable table, say "A", and then set the formula
of dimension Vxxx equal to A, the line takes the length defined by A.
When I unlock the dimension (making it non driving) and then change the size of
the line by hand, the variable table keeps showing that Vxxx has A as a formula,
but the value is different from A.
Afterwards I can't find a way to relock the dimension and make it effectively equal
to A, except removing Vxxx and creating a new dimension as the first time.
Is that a bug? Or did I miss something?
Thanks a lot.