Can gradual blocking of intracranial artery not show as stroke on mri?

An MRI does not show a detailed view of the vascular structures of the brain, it is better for detecting masses and areas of hypertrophy or bleeding. You need an angiogram which shows a detailed map of the blood vessels and can detect any blockages.

A stroke is secondary to insufficient blood flow to the vulnerable area. Gradual vascular stenosis is a warning of critical decrease in local circulation. A CTA or even MRA would be radiological approaches to evaluate extent of problem. Antiplatelet medication or even anticoagulants might be necessary. A neurologist is indicated.