When I start to practice, i usually get a good grip on the strings (using a French bow) however I find that as I play, about 20m in, I start to "slide" over the strings, with the inevitable loss of sound. I appreciate that it is hard to comment without seeing it happen but what are some of the watchouts when this happens? Is it a rosin issue (not enough?), a lazy bowing technique, not enough pressure on the strings, all of the above mentioned? Thing is that when I take a 5m break, the cycle seemingly repeats...