this can happen if the seeds are 1 year or older. thats what ive noticed in my experiments. if they don't pop after 5 days, then your probably out of luck.
Step 1:
STEPONE: Day1
Presoak seeds in a glass ofplain (distilled) water for 24 hours. Do not soak longer or they could rot. At first seeds will float on top of the water. As water penetrates they sink to the bottom.
Within a few hours, seeds will sink to the bottom ofthe container. Seeds found floating the following day are probably not viable.
A small, white rootlet often emerges from strong viable seeds.
STEPTWO: Day2 Use a dinner plate or a small container with a grate. A grate lets excess water drain away. If using a plate, tip it to drain off excess water.
Set a paper towel or cheesecloth on the plate or grate and moisten it with distilled water until saturated.
Pour the water out of the glass and place the seeds on the moist paper towels and cover the seeds with three more moist paper towels.
Drain off all excess water and put the moist seeds in a shadowy, warm location (70°-90°F, [21o-32°C]). The top of a refrigerator is ideal. Check daily and keep evenly moist but not soggy. Do not let seeds dry out or let water stand so oxygen is cut off to seeds. Let excess water drain away freely.
STEPTHREE: Days5-8
Once seeds have sprouted and the white sprout is visible, they are ready to be moved into the growing medium. Seeds that do not show a white rootlet now may never germinate. Do not throw them out, plant them. But such seeds might never germinate, and if they grow they could turn into weak plants.
(above has been copied from Marijuana Grow Basics by Jorge Cervantes)
Good Luck and Overgrow the world!