It doesn't matter if it is a popup or just text in a cell. The only thing that matters is the spelling be the same, including non-visible characters such as spaces.
If there is a category or row in particular that is not working, copy/paste the popup/text from that row of Transactions into the Summary table and see if that fixes it. You can then format it as text to get rid of the popup.
If some transaction rows for a category are getting summed but others are not, try copy/pasting the popup from a working row to the non-working rows.
If neither of these things make a difference, maybe somehow the "numbers" in column D of those rows of the transactions table are actually text. Select one of those cells that is not getting summed. Look at the bottom of the Numbers window. It should say "actual" and have the number. If it says "text", the numeric functions (SUMIF being one of them) will ignore it. Select the entire column (other than the header) and format it as currency. Hopefully that is all it will take to fix it but it may not. One indication of number vs text is that numbers align by default to the right side of the cell while text aligns to the left. If you haven't manually set the text justification, it will do that.
To get a popup back into a cell, copy/paste it from another cell.