Computers can be frustrating when they don’t act as you anticipate. You can anticipate better, however, if you know what they’re going through.
One thing I’ve found with speech recognition is when something is misrecognized the common instinct is to speak more loudly into the microphone — like you’re talking to a person who didn’t quite hear.
This usually makes things worse. Most good microphones are fairly sensitive, and are more likely to interpret the sounds coming out of your mouth more accurately if you speak a little softer rather than louder.
If you notice recognition falling off a bit, speak a little more softly. Picture someone onstage singing so loudly into the microphone that its distorted, and how nice it is when the person backs off a bit. Also think about putting the microphone further away from your mouth — I keep mine about 2 inches away.
Another common cause of recognition falling off is breathing into the microphone. It’s important that the face of the microphone is pointed slightly downward so you’re not breathing directly into it. Picture someone on an outside stage trying to use a microphone in a heavy wind. It’s lots easier to hear when the wind dies down.