Did you disengage the prop latch? This is only present on the Amphibian. You have to pull the prop back to feather and push it back to high idle to disengage the lock, IIRC. Otherwise it won’t develop full power. There’s information in the product manual about it.
Alex on the Discord put it like this:
The short version is:
1) Idle
2) Feather
3) Unfeather