Post the Pronto hex sequence for the power button. Maybe it's a dirty learn and I can tell you how to clean it up. More likely it just needs to be held longer.

Depending on the signal type I can tell you and/or fix the hex code in one of the following ways:

1) Just issue the original learned signal several times in a row and it will act like a long press.

2) A cleaned up version of the signal can be used as described in (1).

3) An extender version of the signal can be made that acts like a long press when issued just once.