Your purchases should sync automatically as long as you're connected to the same Google Account. However, if you find you're missing your purchased Towers or Heroes, please follow these steps:
First, we suggest you try clearing the Google Play cache:
After this, uninstall the game and reinstall it but make sure you have enabled Cloud Save first or you will lose your progress.
If the problem persists after these steps, please send a ticket to our Support Team with a screenshot of your purchase invoices. Here's how to find them, if you need help.