And as the currently top comment says: make your messages public by posting them in the right Slack channels. I had a former colleague literally sabotaging me -- he thought he was getting fired because I was hired, and quickly hated my guts apparently -- by not responding to my Slack DMs which I used to bring a bit more details that I felt could be too much for the channels (logs, error messages from CLI tools etc). I could not progress on important tasks for a few weeks because of him.
After he pulled this stunt twice, I simply started posting in our team's channel where executives are also present. He never dared to delay a response ever again for as long as I was there during the contract.
It's sad that it sometimes comes to this but work is work and I am paid to deliver, not to develop endless empathy towards childish insecurities that result in a literal sabotage of my work.