Strange Issue Cannot allocate memory with free memory available in centos

Question

I’m facing a really Strange Issue in my vps, well Lets me explain im Getting Many:

(12)Cannot allocate memory: [client 172.68.211.52:49792] couldn't create child process: 12: ea-php56

or

(12)Cannot allocate memory: [client 172.68.211.52:49792] AH01223: couldn't spawn child process: /usr/local/cpanel/cgi-sys/ea-php56

* These errors above are from apache error_log.

but when i type free -m it show:

total       used       free     shared    buffers     cached
Mem:          8192       1678       6513          3          0        981
-/+ buffers/cache:        697       7494
Swap:            0          0          0

is there free memory ... i have root access but all of my scripts are running in Another user, Maybe this user can’t use all memory ram but idk for sure, i thought that could be ulimit of this user:

core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 514912
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1000000
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 400046
cpu time               (seconds, -t) unlimited
max user processes              (-u) 1024
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

someone could help me with it?

Thanks in Advanced.