no oil going through filter

[gn dennis]

I was changing the oil in my 1987 Buick. When I removed the oil filter, there was no evidence of any oil having gone through the filter. The filter is a Mobil M1-101. The oil is Mobil 1 10W30. The oil pressure is normal when the engine is cold or hot, the engine seems to run fine, no overheating or unusual noises. Relief value appears to be ok. Why is no oil going through the filter?