Assuming were are talking about a RHP - are they just up or are they consistently up and in to righties? Up-and-in indicates a dragging arm - probably flying open with a low elbow. Up and down the middle - could be too short a stride or just not getting a good “out front” release. A lot of people will say that stride should be about 100% of height, but for me it should be what’s comfortable (meaning what is the player physically capable of at the moment) and what gets the job done. For both stride length and out front release do a search for the NPA (or Tom House) towell drill.
Oh - and JD’s suggestion is really good as well.