Is there a quiet 'click' from the engine bay when you try and turn it over? If so, it may be your starter motor sticking. Mine does the same from time to time-very intermittent with no rhyme or reason as to why. I go to turn it over, 'click', nothing. I turn the key back to 'off', turn it to 'start' and then he cranks and fires up no problem.

Added to the fact you say it has gotten worse over time (you say it took 6 attempts the last time as opposed to the 3, 6 weeks prior), this could be an indication the starter motor has deteriorated further in that time and so is now struggling due to the brushes wearing some more.