Den and Cym are not such species that easy to germinate, I have Den that not germinate until over one month.
But there can be many reasons why they are not germinate. Did you see the seeds swelling, if they are swelling they may try to germinate. If you germinate with seeds that you bought, it could be those seeds have bad quality or the time of sterilizing the seeds is too long and kill them. But you can also buy a simple microscope (I bought one can magnify 40X) and you can check if the seeds have embryos before you flask them.
It's better to germinate with seeds that be havested by yourself, then you know the time period of pollination and have a better idea of the seed quality plus you don't have to kill some seeds during the disinfection.