I seem to recall that if you click on the BH it will show your builders' activity. I sometimes have an obscured build going on, behind other big buildings, and can't spot the builder activity at first. Remember that 'upgrades' use up builders too. 'Hope that helps.
When you start playing you have only 2 builders, more than that need to be bought (with diamonds or special sale offers), so if you did buy an additional one you will have it somewhere doing something