You need to understand that it is nearly impossible to try to figure out why certain specific website fails to load. There are thousands of small things that can go wrong, and each one of them can lead to problems rendering or loading the site. Modern websites are dynamic and tied together with JavaScript.
Things get even more complicated with modern sites that are related to payments: They go their way to obfuscate their function and behaviour and also often attempt to identify malicious behaviour. Unfortunately sometimes "odd behaviour" (such as slow processing) is identified as malicious or bot like as well.
Finally: Websites are not constant and keep changing and evolving. Even if you would A) identify the problem and B) fix it somehow, it could be broken again any moment.
