Hopefully, by now, you have figured it out. If not schlep1967 (above) has the right approach.
My previous camper had the same quick disconnect and I could never remove it. It came with two different attachments. I eneded up leaving the coiled blue hose on it, and then once on, I could never get it off.
My current 5er is one year old now. I keep the blue coiled hose disconnected, hoping it will not freeze on like the last camper did. .... until today. I removed it yesterday and today I absolutely could NOT get it to go back on.
But, here's what I tried and it worked. First, I turned the water pump off and opened a faucet to relieve all pressure on the water lines.
I then sprayed the male and female ends with WD40. That did not do it yet.
I eventually realized there was a metal copper looking cap inside the faucet connection. At first, I thought the quick disconnect had broken and part of it might still be in there. But the ends did not look broken. I then took a small screw driver and pushed inside the female end, right in the middle of that copper cap inside the hole. And sure enough, it was spring loaded, and when I pushed hard enough, it moved!
I did not beat on it, I just simply pushed hard with the screwdriver and I felt it break free. I then turned the water pump back on and tried it again. This time, when I pushed, water squirted and when I quit pushing that center thing, the spring behind it pushed it shut.
I then went inside and turned the pump off again, let the pressure off with the kitchen sink, and then the blue hose attached very easy again, like it use to.
My best guess is, even though I've been putting it on and off multiple times over the last year, somethow, lime or calcium built up around that metal cap in front of the spring, just enough to prevent it from easily moving. By pushing with the screwdriver, instead of the blue hose end, I was able to get more pressure and break whatever was preventing it from moving. I then turned the water pump back on.
I figure, with this camper, I have only 1 hose atachment for that quick disconnect, so, I just decided, I'll never remove it again now. If it siezes shut again with the hose attached, I suppose the only solution would be to cut the blue hose right at the metal attachment, then simply put a small screwdriver right in the middle and push that cap inside the hole free again so that spring and cap inside the quick disconnect will move again. Then I'll have to buy a new coiled hose.
Anyway, several folks above hit the nail right on the head, but had only had partial answers. It takes all 3 steps: Water pressure, WD40, and pushing that spring loaded cap inside the female end (but don't beat it, just push gentle, but firm until it moves. )... good luck!
