If you hit the add boost button (for example when you are about to plant something) It will take you to the apothecary options, and the first boost (fast worker) will automatically be selected. So if you want a different boost be sure to change it to the one you want. This is also the case if you use the boost button between market and edit. It takes you to "my boost" even if you select a different boost it automatically selects fast worker. So again be sure to alter it to the one you intended to activate.
Also, are all the boost for 3 hours? I know Fast worker & Nimble farmer are both 3 hours. Haven't tried Champion farmer or gold miner yet, are they 3 hours too? Wouldn't it be good if this information could be given in the game?