This has been a problem with Starlight Glimmer since day one: she doesn't get enough screentime for the writers to flesh her out. Her introduction in the Season 5 premiere was excellent and gave us a good idea of who she was, but then she went and disappeared for the rest of the season. She was brought back in the finale, but Starlight's story took a backseat to the alternate timelines. What we got was a disappointing backstory that made Starlight come off as a petty womanchild who was throwing an extended temper tantrum. Instead of expanding upon Starlight's character and motivations, her backstory did the opposite. We never learned how she became skilled enough at magic to hold her own against the Princess of Friendship, nor did we learn how she discovered the Cutie Mark removal spell.
Fast forward to Season 6. I'd hoped the writers would expand upon Starlight's backstory and character beyond what we'd been told in the Season 5 finale, but I was sorely disappointed. While Starlight played a central role in both the premiere and finale of Season 6, only two episodes focused on Starlight: No Second Prances and Every Little Thing She Does. She appeared in Spice Up Your Life and The Times They Are a Changeling, but neither of those episodes focused on her ongoing reformation or expanded upon her character. While Starlight did play a major role in A Hearth's Warming Tail, that episode was focused on the story of Snowfall Frost, not Starlight Glimmer. Starlight's redemption should've been a season-long arc that involved her interacting with and learning friendship lessons alongside each of the Mane Six, as well as exploring her past. Because of this, Starlight being thrust into a leading role in the season finale felt awkward because she'd received so little screentime during the rest of the season.
In Season 7, she was the focus of All Bottled Up, A Royal Problem, To Change a Changeling and An Uncommon Bond. She also played a major role in the finale as well as Rock Solid Friendship and Celestial Advice, though the latter were Pinkie Pie and Twilight Sparkle episodes, respectively. So far in Season 8, Starlight has been the focus of The Parent Map, A Matter of Principals and On The Road to Friendship and has played supporting roles in the premiere, The Mean 6 and Marks For Effort. Though Starlight hasn't received as much screentime as I would've liked, I think the episodes she's starred in this season have been excellent. However, I also believe Starlight's character and development will always be hindered by the fact FiM is the Mane Six's show and not hers. While I'm not the biggest fan of Equestria Girls, I'll give the writers credit for integrating Sunset Shimmer into the main cast instead of pushing her to the side like FiM has done with Starlight Glimmer. If Starlight had been integrated into the main cast like Sunset was in Equestria Girls, I think she'd be a stronger character overall and less of a base breaker.