Kindly, may I suggest below two articles for troubleshooting SSL on your sub-domains:
Before moving to Cloudflare, was your Website working over HTTPS connection?
Nevertheless, may I suggest you to check your SSL certificate validity and in case it’s expired, kindly renew it.
Furthermore, if you used Let’s Encrypt or some other, before doing this, kindly enable the “Pause Cloudflare for this site” option from the Cloudflare Dashboard.
After renewing the SSL certificate at your server and testing out, make sure your Website works over HTTPS.
Upon success, un-pause and make sure your DNS records are proxied (as they were).
Do not forget to set the SSL/TLS option to the correct one → Full (Strict) SSL .
In case you do not have an SSL certificate, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to set up an SSL certificate using Cloudflare Origin CA Certificate: