Burton says that they were trapped by an electro-magnetic device. That doesn't explain how such a device could attract human beings, or explain the wind, which would not be affected by such a device.
Steve finds that the explosives they have are missing, and Mark is suspected. After Mark is proved innocent, there is never any explanation for where the explosives went. Mark had no reason to take them, and no giant could have done so without discovering the camp - a much better prize.
The idea that Mark could use his fishing poles in the land of the Giants ignores a big problem: what kind of fish would he hope to catch. Even a tiny fish, like a goldfish, would be far too big for him to catch. He would likely end up being the fish's dinner, rather than the other way around.