Here are some tips.
If there is noise near by, such as from a TV, radio, computer, or other people talking, you must say Alexa much louder than the background noise. It is not a good idea to put the Echo close to the TV or any device that makes sound. If there is noise from one side of the Echo, it would be difficult for the Echo to hear you from that same side.
Try not to place the Echo near a wall. Sound waves bouncing off the wall can confuse it.
When you speak to the Echo, you should face the Echo as you speak. If you are turned away from the Echo, it will have difficulty hearing you because it cannot accurately determine the direction you are speaking from.
Whenever you talk to the Echo, you should not be moving around the room. You should stand still when you speak. I believe there are 7 directional microphones on the Echo. It will turn off the other 6 and only listen to the one microphone pointed at you once it has determined the direction you are speaking from. The blue ring at the top of the Echo points to the direction where it thinks you are speaking from. The direction is indicated by a light blue color, while the other parts of the ring is a dark blue.
There must be a brief period of silence before you say "Alexa." That is, you cannot be talking on the phone and then all of a sudden say something to the Echo. For example: "Hey Richard check this out. Listen to what happens when I say Alexa what time is it?" If you were actually asking for the time, you have to stop talking for one second and then say, "Alexa, what time is it?" If there is no brief silence before you say the wake word, Echo won't hear you.
If Echo doesn't respond well to Alexa, try changing wake word to Amazon to see if it works any better.
Once you can get Alexa to hear you, try the tips by some of other users here regarding finding the best location to place the Echo. Using the app, you can play back recordings of everything Alexa heard. With some trial and error, you should be able to find the best location for it.