How are you setting it in the first place? It must be in so-called text style (\textstyle). You can use \displaystyle \prod_{i=1}^n.
– WernerOct 12 '19 at 22:35
@Bernard Thanks a lot! Please move your comments to answer, I'll accept it.
– JJJohnOct 12 '19 at 22:46
That's not very useful. Lots of comments and answers already mention it. Thanks anyway!
– BernardOct 12 '19 at 22:54
\prod\limits_{i=1}^n– Bernard Oct 12 '19 at 22:25\textstyle). You can use\displaystyle \prod_{i=1}^n. – Werner Oct 12 '19 at 22:35