Before I switched from the ego-c, ego-t batteries to a regulated VV/VW mod, I would press the fire button for about a second prior to taking a puff on occation. This was due to either the battery being low or the atomizer head in the tank needing to be replaced. Either will cause a drop in vapor production. By giving the atomizer head a 'head start' prior to vaping, vapor production could be kept relatively constant.
If you are doing the 'head start' with a fully charged battery, I suggest you replace the atomizer head in your tank.
If you are doing the 'head start' with a weak battery, I suggest you replace the weak battery with a fully charged battery.