Hello :) 

Do you mean that you activated Mana Replenishment Speed, but your Mana didn't replenish at once?

The thing is that this Bonus speeds up the replenishment of your Mana once activated, but it doesn't get you any Mana.

It seems you're having cache problems. 

Because our game is browser based over time your browser do the best it can in order to minimize page loading as much as posible. One way of doing these is storing pixels of website that you visit regularly in you computer. So the next time you visit that page it can be extracted much faster. These is great for standard website. But because games pages changes over time your browser end up displaying images that are old. When these happen, the actual page which should show that you've received mana for battling your dragon can't be loaded in time.

TO fix these issue you've to clear you browser cache. 

On Google Chrome, Opera and Chrome clones

Ctrl+H> select clear browser data, then follow these screenshot

Internet Explorer 11; older versions are similar.

Tools(gear)>Internet options>delete>select Temporary Internet files and website files, click delete, then go to 3 and 4

Firefox

Ctrl+Shift+delete>select cache>clear now

Edge 

Ctrl+H>select cache>make sure you set the date to beginning of time, then delete.