as mentioned, height, antenna can make a big difference. Also while most of the boosters have an internal wifi antenna, if the signal is marginal, you really really really want the phone to be right next to the inside antenna/cradle.
Next, the improvement in phone/text vs. data can be signficant. First they usually are on slightly different frequencies. Next the number of users has a bigger effect on data than voice.
We have the latest weboost in our trailer, along with a omni or yagi on a tall pole. There are places where the phone will not connect to a tower w/o the booster and with the booster we get very good voice/text and very good data (not good enough for netflix, but plenty fast for we browsing) and there are other places where we have no service w/o the booster and get voice/text but marginal at best data. And then there are places where nothing works.
And IMHO don't pay attention to "the number of bars". They are NOT a good indicator of signal quality, and the number of bars will vary based on which phone you have.
Go into the menus and look at signal strength for voice, you want shomething around -100db or less for ok voice/text, by the time it gets to -120db your likely out of luck. Once you get to about -60db your about as good as it gets and most systems will limit power so you won't see a signal much stronger than that. also if your phone show ASU the bigger the number, the better the signal.
Finally, just because your phone won't get any connection, does NOT mean a booster will NOT get you a useable signal, nor does getting a very weak signal on your phone insure that a booster will get you a useable signal.
I've experienced both of the above conditions. We often travel way off the beaten path well away from cell towers. In probably 90% of the time when we have very marginal service the booster gets us useable signals. the other.... well.... cell service is NOT everywhere.
Now if the boosters had a MIMO input....... my hotspot does.