I had the same issues with mine (2017 bike, brand new) 2nd ride in and it was playing up, sinking on it’s own, not staying down…basically doing the opposite of what you want a dropper post to do.
Husband stripped it down completely, I don’t know all the techy jargon for the parts but basically that small metal pin at the top (somewhere in the inside…my OH could explain this much better than me so bare with me) anyway it was stuck half way out, not releasing properly, he literally gave it a bash with a mallet (he got mad with it in the end and was about to bin it anyway) but it actually fixed it! It worked the better then that it ever did from brand new, must of been a dodgy one I think, but it soon started to do the same after a few months, so got fed up of having to sort it. Having said that my OH had the same post on his bike and never had any issues, but had a contact before that one and had endless issues.
Do yourself a favour and just buy yourself a Brand X if you can, cheap as chips, work really well and never let you down.