This problem is rarely reported when Color Correction is used, but it's common with uncorrected prints.
The words of death are, 'it looked good on my monitor.'
And monitors illuminate your photo from behind with their own light. Prints depend on reflected light, usually from the subdued light in homes.
Yikes, is this a guessing game?!
One way to get a feel for how different your monitor is from prints is to order a calibration print.
Another way is to open our calibration print in one browser window and compare its lightness to your shots in an adjacent window.
The top right photo may not look very dark on your monitor, but the levels histogram, right, shows how dark it really is.
The horizontal scale on the histogram goes from dark (left) to light (right).
The vertical scale shows how many pixels in your photo are black (left-most point), white (right-most point) and any brightness in between.
On this shot, the big bulge in pixels to the left is the dark foliage, some of which goes all the way to black. There is usually black in every shot and in this case the strap on her shoulder should be BLACK, which the levels histogram tells us it is.
Almost every shot has white and in this case it's the sun on her T-shirt. But the pixels don't go all the way to white. That means her T-shirt will look gray. She won't like that.
Notice how almost every pixel is to the left of center. This shot would have more pixels to the left of center because the background is dark, but it's unreasonable that almost all of them, including pixels that represent skin and a light-blue top, are crowded to the left.
In the lightened version, the histogram was adjusted by sliding the right-most triangle (representing white) to the left, all the way to the brightest pixel. That makes her shirt WHITE.
The middle triangle was dragged to the left so that about a third of pixels are to its right. Your mileage may vary, depending if the shot has dark objects or light.
The resulting shot will print well whereas the unadjusted version will not.
Adjusting levels like this is easy but it's, erem, the quick and dirty method which gives you no control over regions where you want higher or lower contrast. For that, you'll want the techniques in the washed out section.