It's possible that some friends that you want to add on Line have an option activated that doesn't allow unknown users to add them. You can find the option under More > Settings > Friends > Allow other to Add. If you have this option activated, others won't be able to add you.
To add friends, if you can't find them through the search feature, use the other option available. I recommend you to use the QR Code feature. It will create your custom code that you may send to your friends, and when they scan it, they will be automatically added into your contact list.