Finding Beta Readers can be hard work in an of itself. Keeping them around can be difficult as well. You as the author in need of assistance need to keep a few things in mind when it comes to your Beta Readers.
First, there are degrees of Beta Readers. Some aren't as experienced or may not know what you as the author need. Maybe you're not sure either how to utilize Beta Readers. Not to fear. We're going to discuss this.
Beta Readers are invaluable individuals who offer to read your book before it's been published. They range from offering insights where the book was a bit shaky to editing. Their strengths will vary from person to person.
It's important to educate your Beta Readers on what you need. Don't be afraid to tell them you need advice on where you feel shaky, help working through dead scenes, or editing tips (if they can help with that). Communication is key.
"It's good," doesn't really help you. You need to dig deeper. The whole reason you want people to read your book before publication is to work through any tough areas and to focus group your work. To do that, you need to get your Beta Readers lined out.
This article on Medium should help. It gives a list of questions to ask your Beta Readers. Check out the article here.
You both have the same goal which is to make the book better. With that in mind, work out how best your Beta Reader can help you, so you can get the most out of the experience.
It's also very important to respect their time, after all they're volunteering to spend a few hours reading through your work. Watching over their shoulder and constant reminders will stress them out and make them less likely to help in future. But this is a big deal, so check in every once in a while. Sometimes people genuinely forget.
Respect their feedback. They said it for a reason. Try not to get offended by what they say. Don't argue. They won't want to be honest with you if you do, and it may damage the Beta Reader/Author relationship.
Just be polite, grateful, and courteous even if you don't agree. Make a note of the problem area and come back to it later. Remember what Neil Gaiman said, "When people tell you there's something wrong with a story, they're almost always right. When they tell you what it is that's wrong and how it can be fixed, they're almost always wrong."
Handle the feedback session gracefully, and they'll be more likely to help in future.
And if several sources give you the same feedback, you might need to focus on that bit.
Beta Readers are amazing resources and should be treated with care. Make sure yours know that you appreciate the effort they put into your project.
Also, be sure to give them updates on the book's progress from time to time if they want. They might feel like they put a little piece of themselves into the story as well.