Starting with ESXi 5.1 Update3 TPS sharing is disabled by default.  This was done to close a potential security risk where one could gain unauthorized access to data.  VMware has stated in KB2080735 that this is only possible in a highly controlled scenario and the threat is minimal.  Nonetheless for highly secure environments this option should be left to a setting of "2".

For Non-Prod, DEV environments you may want to consider disabling this new feature.  Most Non-Prod environments have limited resources and thus sharing resources is highly valuable.  The setting is found under Configuration/Advanced Settings/Mem.ShareForceSalting using windows client and Host/Manage/Settings/Advanced System Settings/Mem.ShareForceSalting using the vSphere Web client.  Setting to 0 will disable the feature, allowing full TPS for all VMs.  A setting of 1 will allow TPS on specific VMs only, and a setting of 2 will not allow any TPS sharing.

For settings to take effect you must vMotion the VMs or gracefully shutdown/power-on the VMs.