This happened to me once, on a new PCP I bought. After filling the rifle, it dumped all the air from the reservoir when I bled the line line. I almost took it back, but decided to try it once again. This time opening the valve on my bottle to the gun quickly. I thought the same thing you did, the valve is stuck, so let’s try to “shock” it loose. I did this several times and then filled the guns reservoir to the required bar amount, I think 210 or something. I also cracked my air line bleeder quickly this time and it the valve did seat and hold air. If this didn’t work, and it dumped all the air again, I may have tried to lightly tap the fill valve to get it seat. I wouldn’t try to do that with your fill source hooked up, that could be deadly if you broke fill adapter or whip. Best of luck, be careful, these fill pressures are no joke.