HP35S flag 11 not working

[John Colvin]

The SF 11 command that sets flag 11 on my 35s has for some reason stopped
working. It is used to prompt for variables in an equation when used with
SOLVE. I am using it in conjunction with the F= key. Here is a simple example

LABEL H
SF 11
A = PI * R^2
RTN

After F= H and then SOLVE R, I should get a prompt for A but the SF 11
instruction is ignored and the program goes directly to the equation.
The thing is that the function worked properly the first few times I tested it,
prompting for the variable A, but then it mysteriously stopped working. Does
anyone have any ideas what is happening? The program works fine in the
35s emulator.