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.