There are multiple causes for this issue occurring, though most likely it's one of the following four:
There's a natural limit as to how many videos you can like per day per YouTube account, and you have reached it
You forgot to press like on YouTube
You liked the wrong video
A server-sided issue on YTMonster
One of the solutions above will fix the problem in almost all cases, so we recommend that you try those out (in no particular order) before contacting us:
Double-checking that you have liked the correct video
Pressing Verify Like more than once
Switching the YouTube account/channel, you are using to perform the likes
Should you, unlikely, continue to experience issues while having tried the solutions above, reach out to us, and we will help you out. 😊