Another possibility : maybe the prism is not correctly coupled to the body/lens.

For the metering prism to be rightly coupled to the body/lens, you must first remove the lens, then remove and replace the prism while the lens is removed and then, only when the prism is set in place you may replace the lens ; doing otherwise (installing the metering prism when a lens is already on the body) leaves the photocell uncoupled with the diaphragm.