Bad proxies, they all return 429 (too many requests) or 403 (blocked). I tested it with Google search. Replaced all proxies (you only have I think 5-10 replacements/month) but they were also not returning 200s (OK).
I opened a ticket, and their answer was something to the effect of: "we don't guarantee that our proxies work, it's your problem"
Very cheap - but awful. Once again, you get what you pay for.
My personal theory (not confirmed): I guess the "private" proxies are actually shared with other customers.