Fan working.... But not..?

[corsa245]

Ok, strange one this... My fan/ac has worked no probs, but having driven for a few hours on the motorway I noticed there wasnt much air flow coming through the vents. I turned the fan up to full, and heard the fan spinning up, but no faster air flow, almost like a blockage in the vents??

All the vents are open, and I've tried pressinf to re-direct the flow through the main vents (face), feet, or windscreen, but it doesnt seem to increase the flow?

Then after I came off the motorway in the service station, the flow seems to come back.... Any ideas anyone?

Thanks
jon