for some reason when i attempt to do bljs i start doing it and start picking up speed but then a split second after it started i come to a complete stop how do i fix that

I'm not the most experienced runner here. But a few tips and problems you could be having are: you have a bad camera angle, you are holding the wrong direction, you're not mashing A fast enough, you're not mashing A soon/long enough, or you let go of the crouch button.

For the 50 star door I recommend using Mario Cam zoomed out. Hold directly up for a bit then straight down to the bottom of the stairs, start you BLJ and start mashing A when Mario is about to hit the stairs and if you don't pick up speed immediately keep BLJ'ing holding back and wait to start mashing A again until Mario is about to hit the stairs once more. Then just repeat that.

For 70 star do the same thing but in normal Lakitu cam, and once again up the stairs and then don't go down to the bottom juts hold down for a bit and then do the BLJ, sometimes I just get caught on those stairs as well but you should just keep trying it's not that much of a problem for me at least.

https://ukikipedia.net/wiki/RTA_Guide/50_Star_Door_and_Endless_Staircase_BLJs

thanks so much thags the problem that matched what happend to me was inconsetant mashing since i kept starting to pick up speed and randomly stop