I have a few locations that I want to receive that are coming in only when the conditions are right.
It's quite possible that a preamp won't help that. The best you can hope for is to make marginal reception not marginal. If there's no signal at all, a preamp won't fix it. How far away are the stations you want to hear, but can't?
I added a amp but it did not help and only increased the noise level not the signal.
That's not surprising. Scanners are already pretty sensitive. Adding gain frequently will overdrive the scanner with noise, and cause intermod. What type of preamp did you use? Most preamps have way too much gain, so need some attenuation between the preamp and the receiver.
Noise figure is also EXTREMELY important. In fact, it's 95% of what you should be looking for. If a preamp doesn't have it's NF listed in the specs, drive through, please. The noise figure MUST be lower than the receiver for it to actually do any good. If doesn't matter what the gain might be.
The typical scanner has a NF of 4-6 db, depending. Some TV preamps are about the same, or worse. The end result is, you get "more bars" on strong signals, and weak signals just go away. You may also just get more noise.
I wouldn't bother with a preamp who's noise figure is any more than about 3 db. The lower the NF, the better.
As for gain, anything more than about 8 to 10 db gain is too much. Plan on adding an attenuator between the receiver and the preamp. The value would depend on the gain of the amp.
Also look for a spec called IP3. Third order intercept, that's the preamp's ability to tolerate strong signals without overloading. The higher the number, the better.
As you can see, it can get pretty involved. I'm of the opinion that most people are better off without preamps. They can be more trouble than they're worth if you don't understand what they actually need to do.