In article . com,
Brian Huntley wrote:

My Beto brand floor pump sometimes balks at starting to inflate a
Presta tube. I find if I put my finger against the tube's valve (from
the far side) to increase the back-pressure, the pump catches on and
inflates it properly.

I've had the same problem with my Beto for about 2 years; and have been
the using the workaround you described - works every time.

Luke