Unable to add additional CPU's to a VM.

I have a DELL PowerEdge R720 Host running ESXi 5.5 1474528 and it host 3 VM.

I am using vCenter Server 5.5.0 Build 1476327 Essentials accessing it through the vSpere web client

40 logical processors

20 CPU's X2.20 GHz

2 Sockets

10 Cores per socket

ALL my VM's are windows servers X64, hardware version 10

fileserver     4 CPU's

RDS          4 CPU's

Veeam DAS proxy server      1 CPU

I cannot add any additional CPU's to the third server. I can adjust the CPU count on the other two VM's but not the third.

License is

VMware vSphere 5 Essentials Plus (Unlimited cores per CPU)