longtable is pushed to the next page

I can’t create a MWE with this issue. Seems it’s because of my whole document, as it works normally when I copy the table to a new file. I use these 2 rules:

• For short (10 < rows) tables, I use normal tabular wrapped inside table, with [H] to keep it in place (I really don’t know any other way; some suggest skipping table but then I have to use to add some space).
• For long tables, I use longtable.

I use booktabs on both.

This is the problematic longtable:

This is the same table, but using tabular inside table without [H]. The table jumps to before/above the text We have the following table:

This is my current workaround, using tabular inside table with [H]. This is the intended output, though I have to use tabular, which is inconsistent with my above rule:

In all 3 cases above, there’re 2 underfull warning on the documentclass line.

In a comment to this answer about a seemingly similar problem, it is suggested that I use raggedbottom, but the info about that command is sparse and I don’t know where to start.

How can I fix this, and keep using longtable instead of the workaround?