Elsie's House consists of 72 cards in total. Naturally, the more cards you collect, the harder it becomes to win cards you don't have yet. Additionally, the cards have different rarities, ranging from common (1 shell) to very rare (5 shells).
It just takes a bit longer and a few more tries to collect cards of higher rarities.

If you want a clear overview of the cards' rarities and how likely they are to drop, read the FAQ article "Probabilities."

Please don't give up! Spin the Wheel of Furtune, participate in events and tournaments, complete the Level of the Day... you will find the cards you need!

Good luck!