Have you tried giving it a higher cpu share or reservation?

Share / reservations do not matter.. UNLESS the ESX host is 90% utilized and it has to decide who has priority, thats the ONLY time shares have an affect.

If there are enough CPU / Memory for ALL the VMs and there aren't any limits in jeopardy, you can set one to a million shares and one with 1 share, BOTH VM's will receive identical time slice and memory.