Get your two radios at least 100 feet apart, then test again for your voice
On a GMRS radio you cannot program a repeater on channel 17 - on a GMRS radio channel 17 is Simplex only channel.
Repeater/channel power outputs have nothing at all to do with FRS
You are very confused.
My guess is you are doing something wrong (see last item on the list above) - but Channel 25 should be able to transmit at HIGH power - are you saying the radio is set/showing HIGH power, but only a low-power amount of RF-Electricies are coming out when you measure it, or, does the radio indicate that it is set to LOW power?