Why amount of threads is way lower than set in the program?

So i've set 200 threads on 25 proxies (20socks,5https) and threads are consistently running below 200, often even below 100. What can be the reason for that?
Proxies are private not public.

This is usually because of a lack of target urls or emails. If you're using SER to scrape search engines, it's usually proxy bans or timeouts between searches.