Even though you've mentioned no extraneous spaces, double-check by using LEN on the Q23 item and its apparent-same-value over in the data.
If you're dealing with numbers, confirm that one isn't a hair of a fraction different. One could be 23.5 (either the lookup-value or the data-value) while the other one might be showing on-screen as 23.5 but is actually 23.499999 under the hood. Depending on how you've got things setup, you could check that by widening out the column, or set both cells to Number with multiple decimal places.
If you're dealing with dates, confirm they're both date values. One could be a true date-value while the other might be plain-text that looks like a date. You can check that by setting both cells to General. Dates will show as 5 digit numbers (or 4 or fewer if you're dealing with rilly old dates).
Similar thing with times. Those (true time-values) will show as decimals (the fractional part of a 24-hour day).