Help needed with task declaration

I declare and start tasks using a Cambridge GPSNAV.
I download and view the log with SeeYou. I have had
intermittent results with succesful declarations.
SeeYou does not provide an explanation with the 'Declaration
Invalid'. I have reread my Cambridge material and
am at a loss to understand the problem. Will the movement
of towing to a runway cause the logger to assume the
task has started? I typically don't declare until
immediately before the tow. Any suggestions with this
issues appreciated. Email if you wish.