The service made the claim after apologising to an elderly Shrewsbury patient who was left waiting for more than an hour for an ambulance.
A WMAS spokesman said it was "unlikely to be the last time such circumstances occur".
The Shropshire Clinical Commissioning Group said it would be "working to identify how we can improve services".
In a statement, WMAS said at the time of the call over the weekend, all available ambulances were already dealing 999 calls, but that the delay "was unacceptable".
The spokesman said staff were "left frustrated because there simply aren't enough ambulances available".
The number of ambulances stationed in Shropshire ranges from 11 overnight to 15 during the day.
While funding has been increased in recent years, the spokesman said it had simply kept pace with rising demand and failed to tackle historical underfunding.
He said WMAS "simply does not have the necessary resources to provide enough ambulances for Shropshire".
In February 2011, the service called for an extra Â£1.2m to provide two additional ambulances.
A commissioning group spokesman said providing a service in a sparsely-populated county was "always difficult".
"We do not receive additional funding for providing ambulance services in rural areas and we are raising awareness of the challenges this presents at a national level," he added.
There are not enough ambulances to cover Shropshire, West Midlands Ambulance Service (WMAS) has said.