trouble with combos

im loving the game so far but i'm hoping maybe someone more experienced than me can help with an issue im having.

i cant seem to land some of the most basic combos like nash's cr.mp into f.mp. the combo only seems to land 50% of the time even when im mashing the buttons to try and get the second hit.

what am i doing wrong? how can i improve and are there any videos or tutorials i can look up?

sorry, i hope this makes sense and thank you so much for the helping

Edit: thanks so much for the tips guys :)