Could a mri with contrast fail to identify a pituitary tumor?

Yes: Most pituitary tumors are "microadenomas", meaning that they are less than 1 CM in size. Mris done specifically to look for a pituitary tumor are very good at detecting tumors near 1 CM in size, and identify about 50% of tumors in the 0.3 CM size range, and would miss most tumors smaller than this.

Unlikely: Its 'gold standard' in diagnosis of pituitary growths.

Possibly: If it is a very small tumor it could be missed but usually not if there are symptoms that would allow the radiologist to look closely at the pituitary.