I'm not a prolific raider. But just today, I tried a bounty and attempted an alliance war raid.
I managed to collect the bounty and on searching his fuel gen I got the empty symbol.
In the alliance raid when I searched the fuel gen it gave me the fuel symbol but when I returned home the report said no fuel was raided.
I've heard of a fuel cap and that you can only raid x number of fuel gens but I don't I've hit those levels and if they were in effect it would seem better to have these explicitly explained rather than them being spread in a rumor like fashion on the forum to manage player expectations.
(OS = Win 7, Flash 11.6)
There is a bug in the fuel loot at times. How this bug works is you see a smoking gen, you loot the gen and it says "fuel".. with no number attached to it, then when you get back to the pound, the report says no fuel was looted. This is just a bug on the looters end. We have done a little bit of work on it; I've looted a particular pound and have gotten this message and then had someone else loot that same pound and they received 12 fuel. I know, however that I wasn't at any fuel cap because I went to another pound after this one and looted 35... so it has to just be a bug and it sounds like this is what you encountered on your end. Nothing to prevent it that I have seen.. just some bad luck. My advice? If you loot the gen and you just get "fuel" with no number then back out.
In reference to the fuel cap, I don't think that there is a raid cap as I have even recently hit as many as 7 gens in 24 hours. Maybe there is one for more but I don't know.
Hope this helps