Why cant i sing in falsetto?

im 17, have been taking voice lessons for a few years. ive been singing for a long time and im a tenor. I cant seem to go up into that heady falsetto voice anymore, is this just my voice changing or something else? it just feels really strained and pressured when i try to use my falsetto and i cant hold it as long as i used to. what is going on with my voooooice?