bosch worcester 28i combi boiler problems

i have a bosch worcester 28i combi boiler in the attached garage. I have thermo rad valves on all but kitchen radiator. My problem is when I have the ch on all day and turn it back on the auto setting on the clock timer on the boiler at night...it fails to fire up on either hw or ch in the morning....although the fan runs, it just doesn't ignite and fire up! I have to take off the front cover, and slightly blow into the brown rubber tube going to the pressure switch(?) I suspect it may be that as the boiler has been on all day toasty and warm...when I turn it back to auto at night, some moisture may be settling somewhere with the low temp in the garage causing this, therefore giving me problems day after. Does this sound feasible...and does anyone know of a similar problem...and how can I rectify it? Any help or advice greatly appreciated. Thanks.