There can be multiple barriers to communication. The most common type would be intrapersonal and psychological barriers, such as anxiety, shyness, cowardice, etc. How many times have you chickened out of talking to your crush because you didn't have the guts?
Another type of barrier can be situational. When circumstances dictate impossibility, you just won't have a choice. For example, you want to call a friend, but your cellphone's battery is drained.
Physical distractions are also common barriers. You finally muster up the courage to talk to your crush but someone is playing really loud music, so you can't him/her. 
My favorite barrier is inefficient communication. It's when you have each other all to yourselves, but you just can't communicate properly because he/she keeps interrupting you, or he/she may be very vague with what he/she is saying because he/she is using slang, shortcuts, a different language, or dialect.