I always create separate reverb busses for groups: typically vocals, instruments, and drums.  The reverb in the bus is always set to with 100% Level and 100% wet.  The bus fader is always 0 db.
 
Usually, this is fine.  The problem is that most of the time, a fairly low send (like -12) results in plenty of reverb. In some projects, however, the Send must be at 0 or sometimes way above for me to hear even a small amount of reverb.  This is the same reverb (Lexicon Pantheon) set to the same Present (Medium Hall).
 
Typically, instrument groups are in folders (e.g., drums, guitars, keyboards, vocals).  The groups go to busses (e.g., drums, guitars, keyboards, vocals).  The busses and the reverb busses go to Master.  Even if the tracks go direct to Master, however, I get this same problem.
 
I am going crazy trying to figure out why this is different.  On some songs, I have to end up putting the reverb in each FXBin, which is not efficient for either workflow or CPU usage.
 
Thoughts?
 
 

You say "in some projects." Does this mean this happens to ALL buses and send controls in some projects, and NO buses and send controls in other projects?

Is it possible that you have the effects bus set on pre instead of post? Just offering a guess.

I don't believe there any "known issues" with sends. From just a long time on this board in my memory you are the first to notice a problem. I would check all routing carefully. Leadfoot above has a very good idea too. I would look at post/pre fader settings to see if that doesn't have something to do with your issue.
 
When I use a buss for reverb (an aux send) I use the fader on that buss to add the "right" amount of effect to the master buss. The sends for each instance is then set for a portion that is appropriate to go to the reverb buss.  This can vary widely for either a track or a buss.

As above, it sounds like a pre/post button situation - I can't think of why one would have that kind of level problem if you are inserting the same effect.
 
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It is not a pre/post issue and when it happens it affects all reverb sends on all tracks. All the drum, inst, and vocal reverb levels are low. It is strange, I know.

If a track meters at zero db and the send is at zero, should the receiving bus meter at zero?

If it's happening to all of them, maybe there's unintended quick grouping going on? The bus returns have gain and pan controls. Possibly something is being panned or having its gain adjusted inappropriately. Or possibly envelopes were generated with an initially wrong value.

Is your dry level set to 0% on reverb sends? If it's not you could get some phasing issues with some VST's.
For some reason inserting Reverbs in the fx bin sounds better than on busses in most cases for me.
I know it shouldn't but it does.

I know you said it wasn't a pre/post thing, but just to be curious, which are you using for those tracks?
 
Also, what do you hear if you solo the reverb bus?
 

I always use Post because I want the reverb mix constant when the fader moves.  (Pre would be constant reverb amount.  Post would be constant reverb proportion.)
 
I will have to get back to you on the question about soloing the reverb bus.  I don't remember right now whether or not I have to solo Master to hear a soloed reverb bus.  I know I tried that and the reverb bus just sounded like reverb, but I don't remember exactly what I had to do.