|If you meant to say that you delete them before moving to Junk, move directly to Junk from the Inbox. I'm not sure that will cure your problem, but might be worth a try. |
Another thing to try is to go to iCloud.com, log in, select Mail, and move your spam to the Junk folder there instead if doing it on the iPad. If there is some glitch preventing the iPad from communicating to iCloud what is spam and what isn't, this might fix the problem at the source. I have found Apple's spam filtering to be rather exceptional. I routinely get over 100 spam emails a day that I never see on any of my devices.
All of this is assuming that you are using iCloud for your email. If you are using another provider, you'll probably be better off to deal with spam with their server system.
If all else fails, you can set up filters that you can customize however you wish. This would be a manual work-around to automatic spam filtering. It would work, but take some time to set up and maintain. I would do it this way as an absolute last resort.