I meant ’the images have darker overall tone than I'm expecting'. If so how can it be achieved?
I don't think there's any magic here, except one of perception. See What is "correct" exposure?, and any number of "is this photograph too dark?" questions we've gotten over the years (like this or this). These days, we tend to expect a certain mid-tone exposure and view that as "correct", but there's no inherent reason that this is the case.
My suspicion is that this assumption has two roots. First, automatic-exposure cameras are not smart and make no artistic choices: instead, they just assume everything should be in the middle. So, if you're using your meter's recommendations as a baseline, or otherwise assuming that to be the way things are supposed to be, that's what you're going to get. And second, instead of seeing images printed by the artist, we're used to viewing photographs on glowing screens (often with very poor reproduction of darker tones).
In my opinion, this is a serious loss, because there's plenty of cases where an image is best presented in a high or low key (see this), and it seems like in popular culture we've removed that from our photographic vocabulary. If you haven't, I seriously encourage you to take some time looking into museum exhibits of photography; you'll find that variance in key is quite common. For that matter, take a look at paintings — a medium average tone might even be an exception rather than the rule. After all, in real life, we're often in situations of varying overall brightness.
With digital processing, of course, this is fairly easy to actually do, and processing in RAW gives you a lot of latitude to make the adjustments. And, you can print yourself, or tell the lab you're having do the prints that yes, you want it like this.
With film, there's some combination of initial exposure and printing (or scanning). If you take photographs you want to be low key and have someone else process them, they'll probably be "corrected" to middle tones unless you give instructions otherwise.