Cannot select TM matches with cont + number

Normally, when I get a match from my TM or TMs, I can hit contr-1 (-2, etc.) to copy it to my current segment. 

Sometimes, however, the 1, 2, 3, etc., are not displayed next to the matches, and I have to copy the target text manually. 

Is there any particular reasons for this, and is there a setting I can change to allow me to always use contr + the respective number? 

TIA 

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