Have you tried Sweet Success. I have tried a lot of varieties of burp-less cucumbers as well as some market cucumbers and almost every year the Sweet Success out performs the others and makes for a fairly long time. One year Orient Express did better and actually lasted longer but only once.
You could try staggering your planting which will give you a longer season. I frequently set out some cucumbers in late March and then set some plants out again in late April. The earlier planted ones always start failing way before the later plants so maybe they just produce for so long before they get too tired to keep making cukes.
Bill