Ok, so lately I've noticed that when I go to put my car in first gear or reverse sometimes it doesn't want to go. I have to constantly take it out of gear and retry until it goes in. Sometime I have to force a little (first gear). I haven't heard any grinding noises or anything. I do know it's time for me to do a transmission flush. Is this why I am having this problem? Just wanted to know if I should consider anything else

Thanks in advance!