Just for thoroughness, you have tried using ridiculous settings that no one in their right mind would ever use just to ensure that it's not working at all, right?
i.e. turn the threshold all the way down, and turn the ratio all the way up so that it all but mutes the channel.
If it's still not doing anything then.
1. What is it firing on? There should be a drop-down in the ReaComp controls that determines which channels control the compression. By default, it should be the mains (1 and 2, or the standard stereo output of the channel), but if it somehow got set to auxiliary (a.k.a. channels 3 and 4, which may not even exist), it won't seem to do anything.
2. What are your FX buffer settings? If your buffer settings are wonky, you might not notice changes to your effects in a timely manner.
3. Is the track armed for recording? Reaper prioritizes Effects on playback to keep itsself from grinding to a halt when the CPU is taxed. If you're pushing the limits of your system resources, it may just drop some effects such as ReaComp. Arming the track gives it a higher priority so that this is less likely to happen.