er.. just my 2cents..
one thing that constitutes to monotone singing is singing with very little mouth/lip movement. things like "mumble-singing", or not opening your mouth big enough (er... donno how to explain)
try dropping your jaw all the way, to the most relaxed and lowered position you can manage.. when you sing, does your mouth open within that perimeter or hardly open?
if the monotone sound comes from lack of feel, there's not much i can give tips on except that you need to feel the notes..
one habit i teach on developing feel is to improvise on melodies according to your own style.. it doesn't only encourage feel but works on your confidence..
which is another factor that gives monotone vocals.. not enough confidence to sing it out..
if ppl say you have to put in more air, i assume they think it's not loud enough as well?
if you're practicing do re mi, try exercising your lips and mouth with each vowel...
do - your lips should be unpursed and oval or at least round
try "ohhh" first, then add the consonent for "do"
re - semi open mouth, somewhere between a full jawdrop
try saying "airr", that should be the position of the mouth before adding the consonent
mi - when saying mi, do -not- tighten the ends of your lips. loosen your lips