This could be happening for a few reasons:

1. If you purchased the game on one platform, the license isn't transferable to another.
2. Please ensure that you're signed into the same Play/App Store account that was used when you initially bought the game.
3. If you purchased Bloons TD 5 HD (iPad), you will not be able to download that version to an iPhone or iPod. 

If none of the above apply to you, please submit a support request and we'll get back to you shortly.