I agree with many elements of the previous answers -- it could be due to the wet dough "resealing" and/or to the crust hardening too early and preventing further expansion. Doing a more horizontal slash than a vertical one is helpful to get good "ears," and extra moisture will keep the crust softer for a little longer to get more oven spring.
Frankly, although I did it for years, I don't find the plant sprayer method to do very much -- and if you're opening the oven periodically to spray in the first few minutes, you could be losing significant heat that could actually reduce oven spring. I concur with the steam pan method or using an enclosed pot. Note that pots don't have to be cast iron: any enclosed pot will significantly improve your crust as long as it is oven-safe at the baking temperature.
That said, I think this particular problem is difficult to troubleshoot without observing your specific loaves and slashing technique. Slashing deeper (whether vertically or horizontally) is definitely NOT always the answer and can actually deflate your loaves significantly if done incorrectly. With proper hydration, shaping, and oven steaming, it's very possible for quite shallow slashes to lead to great expansion. (As an aside, serrated knives are also about preference -- if you don't keep your straight-edge knives very sharp, serrated may be a better choice. However, they can leave your slashes jagged on the final loaf, which may not be as pretty and may result in unevenness for shallower cuts. I keep some of my straight-edge kitchen knives really sharp so they can be used for things like this.)
A lot of it depends on shaping and the stage of proofing your dough is in when you bake it, as well as the hydration. If you do a very thorough shaping (i.e., preshaping, bench rest, then very tight shaping of the final loaves), the outermost skin of the loaf may be very taut. Even a shallow slash could be enough to allow the loaves to open up significantly. If, on the other hand, you do a very gently shaping (little or no pre-shaping, trying very hard not to deflate the dough at all), the "skin" will not have the same characteristics and deeper slashes may be necessary. (As an example of this, you might consult the different advice given by Peter Reinhard and Jeffrey Hamelman -- the latter emphasizes detailed and tight shaping and thus advocates very shallow slashing; the former encourages gentle handling when shaping and advocates somewhat deeper cuts.)
The stage of proofing is also critical here: a loaf that is somewhat underproofed will likely have a more taut skin but will also hold its shape better even with deeper slashes. If the loaves are somewhat overproofed, they have a better chance of deflating or at least losing height with deeper slashing. The apparent moisture of the dough and its susceptibility to "reseal" the cut will also change depending on proofing stage.
Again, there are a lot of factors to consider. I would pay particular attention to the behavior of the loaves right after you slash them. Do the slashes spread significantly immediately (indicating a taut surface)? Or do they just remain close together (and thus risk re-sealing)? If you slash more deeply, does the loaf deflate? And if it does so, does it seem to reinflate in the oven, or do your permanently lose height? These observations can help troubleshoot the exact issue. A final concern -- sometimes if the dough is likely to reseal, waiting too long between slashing and getting the loaf into the oven can be a problem. A few seconds should not be an issue, but if you're taking a few minutes to slash and load a few loaves before getting them to the oven, that can be enough time for some cuts to close up again.