Hello,
Since it is not commonly reported, it might be issue specific to your situation, please provide information about following details:
1. Is this only in one zone, if so in which?
2. Are you solo or in party? If in party, are members around? If yes, what are rarity and loot settings?
3. Are you using any addons for looting?
4. Are you using any items/pets for loot/gold?
5. How often does it happen, can you reproduce it?
6. How many free and maximum bag slots do you have?
7. Does both relog and zone change resolve issue?
8. Does it unstuck in any other situation by itself?
9. Does it happen also if you loot items slower?
10. Does it stuck next to specific free slot (like previous slot is some potion, arrow, stackable item etc.)?
11. Does it occur only to stackable items or also singular like gears?
12. Anything else you might find related or useful.
Greetings