The trouble with time travel

I recently re-watched "Harry Potter and the Prisoner of Azkaban." Near the end of the movie, Hermione reveals that she has a time turner, which she and Harry use to go back in time and fix past situations. That made me wonder why we don't see the time turner again in the series. In the next book, Harry Potter and the Goblet of Fire, they could've used it on numerous occasions and saved a lot of grief, especially when it came to Harry, Cedric, or Moody. (I'm intentionally keeping it vague in case anyone hasn't read the book yet.) The time turner would've definitely come in handy during the Hogwarts battle in Harry Potter and the Deathly Hallows.

Therein lies one of the main problems with introducing time travel in a story. If time travel is possible, why isn't it used more often? And why do the characters decide to pick the moment in time that they do pick to travel back to?

The Terminator series is another example where this comes into play. The first Terminator movie actually did a good job addressing the problem, in my opinion. Kyle Reese explains that the time travel machine was destroyed right after he and the terminator traveled to the past, and that's a credible reason why the plot device is used only once. Likewise, sending the terminator back to kill Sarah Conner before her son is born sounds like a logical plan. The trouble comes with the sequels. Now we know that another time travel machine can be built. Why not send armies of terminators? And why choose the two periods in John Connor's life that Terminator 2 and Terminator 3 chose?

The other big problem with time travel is the paradox that it can create. If the terminator in the first film was successful in its mission, then there would've never been a John Connor. In that case, there wouldn't have been a reason to send the terminator back in the first place, so who would've killed Sarah Connor? It's the famous grandfather paradox where you go back in time and kill your grandfather before your parent was born, which ensures that you'll never be born, so how could you go back in time to kill your grandfather?

Whenever I watch a movie or read a book with time travel involved, I can poke holes in the plot. That's why I stay away from time travel in my stories. Well, except for this short story, which you can read on Wattpad. :-)