There are several possible reasons:
1. The player you helped hasn't logged into the game yet.
2. Your mailbox at the Town Hall is full. There may also be no room left for more clovers at your House of Luck. Keep in mind that when you help your friends in their towns, you receive four-leaf clovers by mail and your mailbox only holds 100 letters. When you help your fellow co-op members in the co-op chat, however, the clovers go straight to the House of Luck. The House of Luck can only fit 50 clovers — if you exceed that number, the new clovers will simply go to waste.
3. There's a problem with your internet connection. Make sure that you and the friend you're helping have a stable internet connection. Keep in mind that Wi-Fi networks are generally more stable than mobile networks.