Damp inside shed roof.

Korkyb

My shed roof is damp inside just at the lower corners. 

I applied sealent to the cracks at the edges of the roof felt any all nail areas that could be suspect but it made no difference. 

The roof itself looks in good condition but this morning I noticed that it looks like the photos which I will attach. It's as if the roof felt itself has soaked up water.

The wet area outside matches the damp areas inside. 

What is best way to sort this & which product would you recommend? 

Thanks all. 

Korkyb

Photos as promised

Korkyb

1 hour later looks dry (sun come up properly). 

The damp bit is on the side of the shed that gets less sun. 

Korkyb

This is the other side of the shed.