Yes...nuances... But in an attempt to explain I would argue that "fördröjning" means something that is suppose to happen is taking longer time than it should... Example: The bus is delayed due to bad weather. "Försening" on the other hand is a scheduled "happening" which doesn't happen on time... For example: The bus will be late( försenad) due to the delay( fördröjning) caused by the bad weather.

Fördröjning means delay while försening means to be late.