Like file descriptors and stuff under Ubuntu, you may need to edit some of the /etc/pam.d files to uncomment out the “session required pam_limits.so” line. Files in question are things like cron, su, sudo, sshd, and login.
I was baffled as to why my new shells weren’t picking up a change in limits. conf, until I found this change to PAM’s configuration. Makes sense, now.