Well I spent some time looking at the FCC database for your stations.
More on that later. But first your antenna is very similar to my Winegard
PR-8800 antennas that I use. They are "not" VHF antennas and are specifically designed to work from around 450 to 900 Mhz (UHF)
Now they WILL pick up lower frequency stations but it is not designed to work below around 400 mhz. Anything lower will be a "mis-match" which can cause various problems. Especially if you are using a pre-amp that is also designed specifically for UHF only. As such the pre-amp is going to actually limit front end frequencies below it's designed spec. If they didn't do that then a high power lower frequency can over-load the amp. You would have to insure that your pre-amp is a broad band one capable of amplifying from 60 Mhz all the way up to 800 or so.
The two VHF stations you are trying to get are at present very low power. And their antennas are somewhat directional. If you live to the North to North East direction of the tower, the signal is going to be very weak. Here is the data on those stations.
KNIN-DT Caldwell, ID 192-198 mhz 14 KW ERP
KTRV-DT Nampa, ID 210-216 mhz 17 KW ERP
Those two are way down below your antenna's optimum design freq. And they are pretty weak to begin with.
KTVB-DT Boise, ID 542-548 mhz 948 KW ERP
KBCI-DT Boise, ID 554-560 mhz 89 KW ERP
KAID-DT Boise, ID 512-518 mhz 725 KW ERP
As you can see the true UHF transmitting stations are putting out signals by far much stronger than the VHF stations. Even the weakest of the UHF stations is 8 or more times stronger than the VHF stations.
Add to that one other thing which is the antenna radiation pattern. Here is a plot of the KNIN station's signal.
As you can see, if you live to the North East of that station you'll be at a disadvantage relatively speaking, as opposed to someone living to it's South West.
The hill you are trying to get over is a problem that is compounding things.
If anything maybe a little more height might help? Might only need just a few more feet. It can be critical.
If I were you, I'd get another small vhf yagi and mount that just below the UHF antenna. Then run two separate coaxes from them into the house. Inside the house use a "combiner" to join both to one coaxial feed and send that to your distribution amp. (you might try loosing the pre-amp at the antenna for this)
Also here is a picture of my dual 8-bay Winegards. I have stations in two different directions and a HUGE mountain near by. I get very bad multi-path from that mountain (it's actually a 900 foot cliff face to my North). So I can't use an omni-directional antenna. So I opted to use two antennas, each pointing in different directions to get the signals. These are mounted about 3/4ths the way up on my ham radio tower. Yeah, I'm lucky my hobby has it's perks for my TV viewing pleasure. <grin>
