Ben Flewett wrote:

A number of times I have switched on my transponder
and called for a clearance but ATC have not been able
to see me. I can see my transponder being interogated,
I have good battery power, everything seems to be working
but ATC still can't see me. This has happened to a
number of pilots I know.

Checking ocasionally is a good idea. In the US, you don't have to ask
for a clearance to determine this, but just contact the local tower or
approach guys and ask if they can see you. Actually getting the system
checked as required every 24 months is also a good idea.
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Eric Greenwell
Washington State
USA