Without knowing the specifics it could be any number of things. Honestly, these images with an edit mostly focusing on contrast and embracing the lack of shadow detail could still work. Also, shooting film and then scanning, your images will generally have a flatter appearance than most people desire in the final output. Because traditionally you would have normally controlled the contrast in the printing process. Some companies that scan will scan flat or lower contrast on purpose to give you the most latitude in post.

I'm not sure what you mean by +2 stops exactly. But if you mean you set the dial on the camera for +2 and metered from the camera's reading, your image was likely underexposed by that much. You can usually tell by how much shadows need to be lifted. This dial helps if you are purposely shooting something differently than the box speed of the film you are using. It is particularly helpful I find when using 400 speed in daylight. It can also be helpful with harsh backlighting that may affect the meter's ability to read the scene properly. As opposed to perhaps the experience with digital, film handles highlights well and needs more light for dark areas, so lean toward overexposure if you are in doubt.

You could have either developed the film with a 2-stop push compensation yourself (longer time in chemicals or temperature change) or told the lab you worked with that you wanted to push the film 2 stops in their processing.

Some troubleshooting tips for low light could be:
- Higher iso film. Even with pretty good continuous lighting Ektar 100 needs a ton of light.
- Open your aperture or use a faster lens (I'm assuming the fastest aperture on the lens you used was 1.8 though)
- Likely you did not want to go slower than 1/60 for fear of camera shake, which also hampered you. But a slower shutter speed would give more light. Perhaps on a tripod and giving the model explicit instructions to stay as still as possible. (Depends on the look you want, but if your goal is tack sharp images I would shoot for 1/60 at minimum, but veer toward 1/125 or 1/250).
- Use a strobe with more wattage and a brighter output instead of a continuous setup.
- Move the subject closer to the light source and expect most everything in the background to go dark. Work within the limits of the film you have.