If you are using a trainchinese application then check the 'Settings' - 'Min. number of active cards'. When you update cards the server will try to keep that number of cards on the device (but still limited to 20 in case of a normal user).
If you did not finish learning cards in your application then the cards will remain there (and occupy the space which you can not use for new cards). So make sure you learn the cards you have on the device (or delete them) to get more (different) cards onto the device !!!
Please remember that normal users have a limited space on the application of 20 cards. Only paying subscribers have un-limited space on the device.
Remember that when adding cards to the application the server will first fetch cards form flashcards you have previously selected (or those cards which have waken up). If there are no more flashcard prepared then the server will check for cards in your bookmarked lists, finally the server will use any cards which are suitable for your learning level.