Strawberry problem

I’m growing strawberries in window boxes on my shed
I have two boxes of three plants that have fruited okay
Above these is a third window box with one plant that is three times the size of the other plants
It has a lot of flowers but the fruit doesn’t develop except for two occasions when the fruit was deformed
They’re in a sunny position and well watered
I would have expected the top box to grow the best being more open
The variety is Fragoo deep rose with bright red flowers
Any ideas?
Thanks

 

Maybe it's not getting properly pollinated? If it's more exposed on the top level, the insects might not like hanging out up there.

 

Foliage looks a bit poor, just wonder if its virused.

 

Agree about the foliage. I'd be tempted to take those top plants out, clean them up, give them a general purpose feed, pot individually and see if they improve. No personal experience of Fragoo but worth a try.

 

the red flowered ones are hybrids with another genus, potentilla, or descendents of those hybrids, and are not as good at making fruits. They do still set but are more fragile