We just checked to ensure that Proxylist.co's Scrapebox proxy test engine is working as usual. I have tested it (I own two Scrapebox licenses) to be working well.
Based on your Proxylist.co account, you have 200 HTTP proxies, but it doesn't mean that there will be 200 Scrapebox proxies for each country or another selected filter.
For example, for United Kingdom there is only 4 up to date scrapebox checked proxies. So the main criteria for proxy is 'Last checked', that means this proxy is up to date. If you changed the filters/settings, the latency, or country, or something else can give old proxies, which will not work correctly.
As suggested earlier (which works very well for majority customers), please do not use other filters or settings other than Scrapebox filter when you plan to use it for Scrapebox.
Then, finally, do not use Scrapebox proxy tester. It is not accurate. I see you keep on testing the proxies using Scrapebox proxy tester, which is not recommended. Proxylist.co's own scrapebox proxy tester works pretty well.
Hope it helps.