I've had this problem once before, on a website using Store on 1&1, where the checkout didn't redirect to PayPal, but got hung up on the final step and just refreshed the page (see original question/answers here: Expresso Store orders suddenly not getting past checkout 3 to Paypal). The problem was sorted after contacting the host, but they didn't tell me what they did to solve the problem.
I have another website, which is hosted on Clook, and for only some users, Store is getting hung up on Checkout 3 and not going to PayPal. The page just reloads. I can't replicate the problem, and neither can my client, but he's getting several complaints from people who can't buy on the site. I am not getting any errors on the page. ExpressionEngine v2.5.5 Store v1.6.3
Anyone have any ideas?