We all have days where we stumble over copy, and can't seem to find our groove when executing a commercial or narration script. It's easy to get frustrated and beat yourself up. The best thing to do is walk away from the project and come back to it later. Too often, talents seem to force a performance when they're having a bad day. I've been guilty of this myself, and it's a waste of time and energy.
The problem is that you can hear the irritation in your voice. You're trying to record a friendly and relatively upbeat commercial, but it's slowly starting to sound like you're pissed off. At this point, you're beginning to waste your client's time, and you're likely going to have to re-record it anyway. This isn't the best method for keeping a client for life.
If you're having a lousy day and you can't seem to get a performance right, take a walk or a nap and come back to the project later. Sometimes all you need is a reboot! Of course, this only applies to freelance voiceover jobs, where the client has given you a few days to complete the project.
If this is a talent agency booking, 90% of the time, a client will direct you live while you record the project. Typically, you'll have an hour to come up with a stellar performance. So, if you're not feeling it right away, take a deep breath, pay attention to the client's direction and take it slow. You don't have to get this done in ten minutes!
If you're working primarily as an independent contractor and working with clients direct, there will be plenty of times where the client will give you some direction on how they want their script to be read, and then they'll dismiss you to go off and record on your own. This is an example of a situation where you can always record the commercial or narration later if you're not feeling it.
The goal is to always have a client for life, so if you're not having your best day in front of the microphone, take a break and come back to the project later. You'll be happy, and most importantly, your client will be satisfied.