How can I correct my pitch problem?

I am very weird I think. When I am warming up and singing only vowels I am almost never flat or sharp but when I add consonants I have problems. I would say I am flat 20% of the time when I am singing real words. I'm not sure why. My teacher says it is because I am concentrating to hard on the consonants and not the vowels. I get so confused. This sucks and I really want to correct this situation.