Is the down smash or up smash getting shielded? If so it’s probably because you’re not getting enough hits of the up air. If you only get 1-2 hits, it won’t be true. You have to get 4 of the 5 up air hits, I believe. The combo counter in training mode should read 5 for fair 1 -> dragdown up air (4 hits) -> down smash.

Also, keep in mind that the frame advantage after dragdown up air is only high enough after a certain percent. If the enemy is at a low percent, like 40%, the frame advantage is less than +10 so down smash won’t be true and they’ll be able to shield just before. The enemy typically has to be above ~90-100% for you to be at least +10 after the dragdown up air. So if the smash attack is getting shielded, then you might be doing the confirm at too early of a percent.