Trilling

I have never been able to trill. I do not know why; I just know no matter how hard I try I am just unable to trill. Is there an effective alternative to trilling? I do not have issues with any of the other warm-up routines, just trilling. I can roll my lips without sound (just air), but attempting this exercise with sound or tone is not possible for me. You may say "you just need to practice", or "you are not doing it correctly". Trust me when I say I am unable. I took one-on-one vocal lessons for approximately a year. My vocal coach eventually just had me do the exercise with my lips closed. Can you give me any advice? Thanks, Paul A 

Any semi-occluded exercise is a great replacement with many of the same benefits! Straw singing is the most popular. You can also try a sustained VV, ZZ, NN, MM or NG.