Check your cargohold. If it's full, then you can't loot anything (minor duh)
If that's not the problem, then I can't help you.

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The Titan Flies Like A Cow
"They're talking from here..." - Freddy Mercury '86

Juni: Oh yeah?!
Trent: uh, huh.
Juni: Oh.
Trent: yeah, great.

TLR: You can check out anytime you like, but you can never leave.

Not sure how new you are to the game, so forgive me if my answer is not at all what you are looking for.

My first guess is that your cargohold is full, as previously mentioned. If you are attempting to tractor stuff in and the computer says "Tractor Failed" or something to that effect, then it's most likely a full hold.

However, perhaps (and again, forgive me if I'm wrong) you are really new to the game. You have to shoot the small moving rocks or debris. Although there are many pieces floating around, there is only a specific type that can actually be shot. These will cause your targetting reticule to light up. Use only one of your guns (press the 1 key to fire only one weapon) to avoid accidentally destroying any resource you might find. Not all debris/rocks will produce a resource. Some minable fields produce more stuff than others and you just have to search around and be patient as you look for those areas that have a higher percentage of resources. Some areas will produce a resource almost everytime you blast a rock/debris whereas other areas will require you to shoot 10 or 20 or more rocks/debirs before you find anything. You will know when you find a resource when you blast a rock or debris and a second later see a "lootable" item float out. It should be surrounded by a little white box. This is easy to miss if you are flying too fast so be careful. You can also set your scanner to "Loot" and any items you discover will appear in the list. This is your cue to hit the tractor beam button (default 'B').

Ok, so there's a brief guide to mining, which may or may not have been helpful to you, depending on how long you have been playing.

Cheers.

Wap

Yep, check the nav map and click on the little crossed over pickaxes icon, that will highlight areas in which you can blow up those little rock thingies and have a chance of getting something... good starting place in the debris feilds in new york or perhaps the silver field in colorado (altho it is very rare to get a decent amount here)

Also check area discriptions in the nav-map (click on the asteriods, then the ? icon) as it sometimes gives hints on the abundance of the resource there

Sometimes the glass is half full, other times its half empty. Usually its just twice as big as it really needs to be

Are you sure you've actually found anything to tractor in? Sometimes you only find ore in maybe 1 out of every 15 or 20 rocks you blast.

Jose Chavez: "Trent! It's good to see my kind of scum."

The amount of times it takes to tractor something in depends on the rarity of the item in question( exept in reinland). (li)

what rocks contain loot?

The small ones floating by.
You can aim your mouse and the pointer changes to a targeting thingy...

also those in the icefields?

Contains spoilers. Moved!