I have found in the past, if the agency cancels you more than 2hrs before your shift starts, that is classed as ok. Less than 2hrs they give you 4hrs pay.
I have been a full-time agency driver for 15 years now and luckily I haven’t had that problem for years. Maybe for you it maybe time to have a change of agency, it depends what area you come from if there is a demand for drivers.