Why do I keep getting "Hack acquired no items" in Ingress?

As far as I know when you hack a portal there is a certain probability that you get nothing. As well as there is a probability to get a good loot.

My observations:

• Hacking enemy portals has a much higher "no item" probability, my estimates would be around 50%
• Hacking the same portal several times in a row, significantly increases the "no item" probability. I had an opportunity to hack the same portal 5 times within 30 minutes (this by the way causes burn out) 2 days in a row, usually at least 2 of later trying (3-5) did not bring me any items except sometimes a portal key.
• Portal key has a different "acquire" probability than items
• I believe if while hacking you jump out of the portal's reach you also get a "no items" message, but I have to test this to confirm

They are doing this to combat location spoofing. If your GPS is glitchy or jumps around, this can happen. If the anti-cheat measures set in you will not be able to do any action (hack, deploy, recharge, link, etc.) for a while.

Speed limit.

I get this message on trains when it reaches a certain speed. Then I can't hack, attack, deploy, recharge, drop... and so on. I guess this is to prevent car drivers from playing dangerously.

Once the speed drops under that limit, something Iike 1 minute is required to regain abilities.