You only get achievements at certain points – as documented on the beta page:
Achievements are reached when certain conditions are met. Achievement “tests” are not done “IN GAME”, but only at certain game situations (enter/exit shop, end a level, end of game…). Achievements are collected out of the game as “life time achievements” and are as such available “for ever” once you enabled them. The achievement rewards provide (in general) better start conditions/options for all following games. Most achievement rewards are cummulative.