Orange cukes usually means past ripe, past ready to pick. Perhaps your definition of "halfway mature" needs to be adjusted for the variety you are growing? Size isn't the most valid indicator.

On the other hand incomplete pollination and/or fruit abortion are other possibilities too but that normally shows up when the fruit are still small. If you are convinced you have adequate pollination then fruit abortion would be the most likely.

Stressed plants will abort fruit. There are numerous causes for stress - over-watering, lack of proper nutrients, root damage, weather, too many unpicked fruit on the vine, etc.

Dave

yes, cucumbers are orange when ripe. nobody eats ripe cucumbers. just pick when green at a size you like.