Use proper pollination techniques and fertile pollen. Pollen that has been handled or stored improperly can result in some issues getting the seeds to come along at the right pace.
With that said, the most important thing is to just let them go. Don't pollinate a plant with the intention of it having any other purpose than reproduction. This way you can let the plant tell you when it's ready - just like all cannabis plants mature at different speeds, seeds will often mature at different speeds.
When the seeds are ready the calyxes will begin to split open and you'll see a slightly brown seed. When you see this happening to the majority of your calyxes not only on the primary cola(s) but a node further down - you're ready to harvest.