1. You aren't aiming at the center of the pitch.
You think you are, but you aren't really listening to the pitch you want to match. You need to learn to be aware of where acoustic instruments are... not swimmy instruments and not bass. Listen for the piano or acoustic guitar. Listen for the CENTER of the pitch in the track- not the over or undertones. Or if you're trying to sing with an acapella group, listen to the center of the group pitch.
2. Your can't aim at pitch.
Your ear may need some training... you may KNOW you're flat or sharp, but can't seem to make your voice 'go there'. Your ear, vocal cords and brain need to actually grow some neural networking and muscle coordination. And YES indeed, it can happen... all you have to do is be willing to do the work.
Get some ear training with an intuitive vocal coach who does this, or at least with a friend who knows if you're on pitch or not. This needs to be a slow, patient and persistent practice situation where you can be corrected if you're wrong. If you don't have access to a vocal coach or other knowledgeable person who is willing to take the time necessary for you to practice aiming... a lot... here's where some of the pitch training software out there may help.Still there's nothing like another informed human ear to confirm that you're getting better.
3. You are not letting sound vibrations travel far enough.
You need to become freshly aware of where your voice is coming from. You need to learn how to use breath and throat channel configurations to allow your voice to travel farther through your alternative resonation zones. Simply put, you need to lift the ceiling for flat pitch to let your voice 'fall up'; relax the floor for sharpness to get better chest voice resonance and let your voice settle down. Your vocal sound should feel like it moves through different places in your throat and mask for different pitches instead of feeling like it's coming from the same place. Then just the intention of singing a certain pitch will put you dead center on it! This one you might want a vocal coach to help you with.
4. You can't hear your voice accurately.
When this happens in the studio you might find great help by taking one headphone half off so you can hear your voice acoustically and take some of that aural disconnect away. When it happens on stage, you either need to have more of your voice in your cue mix or your in ear monitors, or the band needs to turn down.
5. You're pushing too much air to aim your voice accurately.
Back off! Hearfones can help you learn to back off excessive air pressure, and also to hear what your voice is really doing rather than what you think it is.
Hope this provides an a-ha moment for some of you...
Let me know!