If you collect an item from a group of item boxes, you can’t get any more items from that group. If you were to go backwards and grab items from the end of the track, then go forward into your current lap you can’t get any items for that lap at any group of item boxes.
Okay, while I do sorta a little bit disagree with this, I think something with this is causing you to not get items sometimes, even when playing normally.
I was racing with two other people, and on Pine Valley track 2 we had a few times where we couldn’t get items from the first two rows of item boxes if we took the right (and not the ramp over the river) near the beginning. This would also cause it to not give you items at the second item row either. Here’s a clip of one of those times.
And I don’t know if this is related, but someone posted this in #tower-unite on the Discord.