I think I had this glitch a few times. When you can't hit something within your own radar range when you are near your Howies, something is wrong.
But for me, the scavenger "no more scrap" glitch/ignoring certain scrap pieces glitch is more common.
Let's say I am playing an IA mission and I have 8-10 scavengers gathering the remains of an enemy mass-attack. More attacks come, and most of them get destroyed. Most of the scrap is cleared, and there's a few more pieces, and my 2 scavs are ignoring it and either collecting other scrap or reporting "no more scrap". So I select one of them, point at the ignored scrap piece and press SPACE. The scavenger drives over to it, and then ignores it. I DO NOT TOLERATE INSUBORDINATION!!!

I have him pick me up. (if he were to somehow refuse to pick me up I'd snipe him!) and then I'd get in the scavenger and run him over. I would then pick up the scrap myself and drop it off at recycler/silo. Job done. Nothing to it.

I think this has to do with the secret agreements made between scavengers. No two scavengers will ever go for the same piece of scrap, although they commonly fight over scrap fields.

So I think the scrap gets "marked" as belonging to a sacvenger, and that scavenger gets destroyed or something odd happens that causes that scrap's "marked" flag to remain "true", and so other scavengers refuse to pick it up. I can't be entirely sure though, but my speculation could be somewhat correct.
