Play the other version of this TD. It has a chinese name. All the cards are for free and u get to customize your deck from the get go.

Thanks for the info, but that answer didn't really help me. xd
My question was why I sometimes don't get units when I already own a big amount of units and e.g. have 12/12 on the field.

A bit late response, but when you open packs ingame, you "draw" cards from your deck, that can be seen at the top right. So, if you exhaust your deck from cards of certain rarity (drawing them and keeping them on the map) you may get nothing from a pack.
The "fix" is to sell/feed some of your cards to get them back in the deck.

Ah okay, thanks for the response!