Menu sub item not displaying sub items beneath it

One of my menu sub items (ex: 2020) is not displaying its sub items (ex: 2001RL). The other menu sub items (ex: 2019) and their sub items (ex: 1911OC) are identical and working just fine.

After several rounds of troubleshooting, I think it has to do with the position of the sub item (2020) within the menu. When it is the sub item at the top of the drop-down, it does not show its sub items (2001RL). But when I put a sub item above it (ex: 2021), the sub items appear (2001RL under 2020).

Can someone help me fix this? Thanks!