i'm a web developer, and I made some spa bookings website for some of my clients.
Now their biggest problem is to increase bookings online.
Is there a method to increase the spa online bookings?
It's really no different than any other website. You need to drive relevant traffic to your site, if your site is good, and the product is priced right, people will sign up. I'd start by looking at your conversion rates, and comparing it to industry standards. If you have a lot of traffic, but no one is converting I'd work on your copy and tweaking the site. However, if you have low traffic, but customers are converting you will need a way to drive more traffic to your website.
You have to find where the friction is and do your best to reduce it. To find that out you'll need to make a research — qualitative or quantitative — and uncover those pain points. It may be that people are just avoid places like spa and it has nothing to do with your website. But having actual information on your users' behavior, needs and goals should give you enough initial information to form a hypothesis and test it.