FCGId 503 service

One of my domains that I transferred from a CentOS server to a Debian 8 server ran as it used to with FCGId as the PHP script execution mode with a Maximum Memory Allocation of 128M on both servers.

I subsequently began to get the following error in my logs on the Debian server, and web pages displaying a 503 service temporary unavailable notice.

[Sat May 13 13:37:24.613820 2017] [fcgid:warn] [pid 19176] [client xxxxx:58336] mod_fcgid: can’t apply process slot for /home/xxxx/fcgi-bin/php5.fcgi

I increased my Maximum Memory Allocation to 256M from 128M for this domain that runs an e107CMS, but the error above appeared after a while. I changed the PHP script execution mode to CGI wrapper and don’t have a problem.

Would appreciate any advice on how I can troubleshoot this as I’m curious as to why this should happen with FCGId?