You have to collect spells before you can use them.
The 1st spell appears after you have defeated one word.
If using the spell would provide no benefit then the spell will not be cast. (Eg: Casting ❤ when you have full Health.
Okay, I thought there was a bug because I acquired multiple spells across all of my playthroughs but have never been able to trigger them. Perhaps some sort of reactionary "This spell won't do anything at this time" or something might help, because I've never been able to get any to work. Anyways, interesting game! I'll try the spells again later.
I cannot reproduce this issue.
If you collect a spell you can press number keys 1, 2, or 3 to activate them. If they cannot be cast there is a noise and a red cross will appear over the spell. The numpad keys will be affected by numlock so you may want to use the main keyboard numbers.
Example: This is the randomise spell being cast by pressing the 1 number key.
All right, I now see that the downloadable PC version works just fine; it's the browser version that's at fault (either that or Edge 99.0.1150.39 or Win10 21H2). Thanks and sorry, I should've tested that earlier!
I tried Edge on Win10 and it still works for me. Don't know what's going on but glad the desktop version works for you.
We'll see if anyone else has the same issue.