I've grown slicing cucumbers in my garden for about 5 Summers, I've decided to try pickling cucumbers this year as well. I started my seeds for both variety in a glass of warm water for 10 hours, then transferred then all to a wet paper towel in a plastic bag. ALL the slicing seeds sprouted within a day so I transferred them into plugs, and they ALL came up within a day. It took the pickling cucumbers 2 days to sprout half the seeds, the rest did nothing. I transferred the sprouts to plugs and 2 days later they're still not coming up. Are pickling seeds just harder to start? I can't imagine what I did wrong when the slicing seeds did excellent the same way.
I'm assuming you purchased fresh seed for the pickling cucumbers, but I couldn't find any information on the difference in germination rate. However, it's possible that the pickling seeds need it warmer to ensure good germination and growth. Keep in mind that with most pickling cucumbers the time to harvest is substantially shorter than for slicing. If you aren't using a heating mat for your seeds that may be a contributing factor. The other suggestion I have is to contact that place where you purchased the seed and ask about germination rates and possible replacement of your seed packet.