The amount of light lost through a piece of glass depends on several factors:
- Angle of incidence of the glass to the sunlight. This controls whether the light is reflected or passed through.
- Which component of sunlight you're interested in. UVA can penetrate glass, but UVB does not significantly penetrate most glass (source). The component of sunlight that plants generally use for photosynthesis can generally pass through glass at a maximum of about 90%.
- If the glass is dirty, it will not pass as much light.
- If the glass is tinted or coated, it will not pass as much light.
- Thickness and general composition and type of glass.
You're facing west, so you're only receiving half a day's light already. Assuming the windows are vertical, your minimum loss to reflection will be right at sunset when the light strikes the window at closest to 90°. Prior to sunset, every moment closer to noon you lose more to reflection because the light is striking at a greater angle. If you're in a city office building with reflective-coated windows that only get washed a few times a year, then they're almost always going to be dirty and losing light.
If you are in an office building (i.e. can't control the windows), and the windows are causing a lot of light loss, you don't have much choice but to relocate the plant to an environment where it can get more light (or get a different plant that is more tolerant to low-light environments). If you are located adjacent to a window, you can try placing the plant in the direct sunlight through the window to see if it gets better.
If you are in a setting where you can control the window (e.g. home office):
- Make sure it stays clean.
- If you have window screens and you generally leave the window closed, consider removing the screens -- these reduce light transmission.
- As noted above, move the plant closer to the window, possibly placing it in direct sunlight.
- Consider moving the plant where it can be in a south-facing window so that it gets sunlight for a longer period during the day.