@BeatriceBootes. I was going to say this is not the best place for hardware advice as a troll might brick your set up. However, I trust the advice of @Drathek.
If you dont spriteclick your directional slash can miss when the target moves a tile even though they may appear on the same tile but their sprite is just moving to the next tile.
Otherwise even though unlikely, your problem may be because of your mouse. I had a similar problem once in another game before realising my mouse left click was a bit broken and didnt register every click / when holding and dragging it would let go.