how much ram is currently allocated to each VM? How many VMs do you have? Which hypervisor are you running?
Depending on how you have things setup, the memory may be able to be changed without shutting down, but typically the VM has to be powered off for the memory increase to take place.
A script like this may be able to help (if you use PowerCLI)...
PowerCLI: Script to Schedule Memory and or vCPU up/downgrade | ICT-Freak.nl