According to Fightful, FOX is disappointed with the ratings WWE is pulling in for SmackDown, especially compared to the numbers they expected when that big-money deal was made for the show.
It was said at the time FOX expected around 3.3 million viewers for the show, and the premiere featuring Dwayne "The Rock" Johnson drew an impressive 3.9 million viewers. Unfortunately, it's since plummeted to around 2 million viewers each week, a big drop from what the network was expecting/hoping for.
FOX is also said to be unhappy with SmackDown being used to promote NBCUniversal's Peacock - the new home of the WWE Network - streaming service, especially when they have their own platforms (Tubi and Fox Nation, for example). Believe it or not, FOX owner Rupert Murdoch is also said to be concerned with his portrayal in The United States vs. Vince McMahon TV series that will delve into the media portrayal of the WWE Chairman during his infamous steroid trial.
McMahon and WWE President Nick Khan are said to have recently met with FOX Sports CEO Eric Shanks and head of production Brad Zager, and certain promises have been made (like an upcoming episode of SmackDown taking place in Los Angeles instead of RAW due to that being the home of FOX Sports).
It was also recently reported that FOX wanted WWE to make a greater effort to sign CM Punk, and it's fair to say that things definitely sound strained between the companies.
What sort of impact this will have when it comes time to renew that TV deal remains to be seen...