Hi!
I believe the reason why you have more success with the first method is because you might do the second one wrong.
Now, Shai wrote this
http://smashboards.com/threads/shai-drop-update-detailed-guide-frame-data.322062/
guide on all the convenient ways to shield drop.
I'll try and condense some of the information:
The first method you used can be done extremely quickly, within 2 frames, but requires extreme precision which may need a lot of practice to pull off consistently mid-match.
The second one is significantly easier as you only have to angle your shield towards the right or left without rolling. Angling it to a position similar to forward tilt position is about where you want to be - you can go further but then you would have to do it slowly as it would trigger a roll otherwise, also you should be used to doing the forward tilt motion.
You then slide your control stick downwards to the bottom left or right corner, matching the direction you tilted your shield. You should be able to do this at any (human) speed and it's significantly easier to learn as the motion doesn't have to be as precise.
You can even use this to shield drop method while running!
Since you already have the stick in a forward position, you can simply slide down to down left or right, press shield and you should drop from the platform immediately which allows for really nice movement and is a good way to practice the downwards motion.
I recommend light shielding because you drop so quickly that you may already have dropped before you press down on L/R and accidentally airdodges.
Edit: This is also useful if you want to use the same button for shielddropping and l-canceling .
My white Japanese (main) controller has a problem with recognizing the stick, when I tilt my shield right and then down-east, I spotdodge instead of drop which doesn't happen with any of my other controllers.
You can check this by tilting your stick to the right, then pressing start and inputting the down-east or west direction and pressing start again so for the game you did that motion within 1 frame. You can also do this to explore the area in which the shield drop is triggered.
Hope this helps