Everyone thinks that they're your only client

No matter how much or how few work you do for a single client while having multiple clients, they all expect that you can just drop everything and do their work immediately or answer their phone call or email immediately. The truth is that when your independent and rely on multiple clients for income that's just impossible to be available to everyone at once 24/7, even if you have an office that you work out of the majority of the day every day, sometimes you just get tied up on the phone or over Skype with someone.

There's a very thin line between keeping the customer happy and the customer just asking for too much. Some people just plain ask for too much attention. You have to find a balance between getting back to your clients in a timely matter, being sure not to ignore them, but also making sure you are not ignoring another client in the meantime. Generally you figure out how to deal with each client individually, and juggle your time by being honest with them and explaining that you are unavailable because you are busy with another client, but will get back to them as soon as possible. You also can't let it drive you crazy and in the long run, you have to figure that you just can't always work perfectly with everyone.