I would assume you are inputting the attack too soon, and whiffing during the tech animation's invincibility, up until you said they are able to buffer things after teching and evade your punish attempt (obviously you're super late in the latter scenario).
Techchasing is fundamental. Any time you create a techchase opportunity, you need to have an exact understanding of your opponent's options and find a way to cover it with an appropriate hitbox or grab. Know their getup animations to aid your reaction time and accuracy with/to visual cues. Know your knockdown percentages.
With a grab, you get to decide almost exactly where someone well tech, and when. You have to have your timing down with covering techchase options. With platforms, your opponent's options are restricted, and you have to know what this entails and decide what the best punish would be based on his percent and location on stage relative to the platform/ledge/other platforms.
Watch EXACTLY how PPMD uses shorthop (non-fastfalled) to cover (usually) every techchase option fastfallers have in the bottom platform. Marth's aerial mobility is just good enough to allow for 100% coverage if you can get in position in time.
At low% you're going to want to ff the uair and hit him with a tipper and then utilt. As his percent climbs don't yolo the utilt, you'll whiff (early); be precise and time it. Pivot as necessary to keep your combo going, but obviously try to end with utilt facing center stage to set up for a free dair or bair gimp.
It looks like you just need to work on techchasing more. Techchasing means a free hit usually, unless it requires a hard read and you goes wrong lol (consider not only likelihood of where he will tech but also reward potential for each option). But think strongly about your punishes. NEVER let them techroll to the ledge and go unpunished.
. I don't know though, but I'm sure you can figure it out.