Some words in checkout don't translate using translation file

Hi all,
I’m having a problem for a while now with some words in the check-out of the woocommerce plugin. I translate those words from english into dutch using the ‘woocommerce-nl_NL.po’ and ‘.mo’ files (using poedit), but the translation doesn’t appear. I’m sure i’m using the correct files for translation, because the exact words appear there, like:
-proceed to checkout
-update cart
-apply coupon
-…etc

Anyone any idea on why translating these words doesn’t work?