The key is patience, sometimes the downloads do take time to start.
Check the %age health of the file. When selecting downloads always look for them in Green as green means that you may be able to successfully complete the download.
Secondly, sometimes the people uploading may not have the 100% file with them.( I am stuck on one download at 96 % with all seeders being stuck at 96 %

)
Try it out and I am sure you will get it or re search for it and then start the download.
P.s : I have answered this keeping in mind that you may be facing this problem with only one file.Please correct me if I am wrong.
Thanks