Had this issue on plesk, when disk space is used up by a client and you increase the space in the control panel, sometimes it doesn’t go through to the system.
Login via ssh and su to root, type these commands:
# quotaoff -a
# quotacheck -am
# quotaon -a
If that doesn’t work and the plesk quotas are still not coming through, then you have to edit the quota through ssh as well.
Check current quota:
# edquota username
Edit hard disk quota
# edquota username
increase the column under “hard” and your done.