Why couldn't the Jedi throughout the Star Wars prequel trilogy sense that Senator Palpatine was a Sith lord given their regular interactions with him?
Just to add to the previous existing answer: The Jedi Temple on Couscant sits ontop of a Sith Shrine.
The Shrine in the Depths was an ancient Sith shrine situated at a Force vergence on the planet Coruscant, constructed before the rise of the modern Galactic Republic. The Jedi Order later constructed the Jedi Temple over the shrine, hoping to neutralize its dark energies, and occasionally nullified dark side artifacts inside of it.
While I don't believe there is any definitive proof stated anywhere in canon that this contributed to the Jedi's inability to see/feel what was going on with Sidious, it is highly speculated that it played a role, especially considering Sidious spent a whole lot of time there once the Empire ruled in order to harness its power!
As another point of interest, a new TV series titled, The Acolyte is debuting next year. It is set at the end of the High Republic era about 100 years before The Phantom Menace. It has been stated by actress Dafne Keen that the story is about how the Sith were able to infiltrate the Jedi Order. So there may be more insight to gleam from this series in terms of how the Jedi transitioned from the more glamorous emotionally available Jedi of the High Republic to the more minamalistic and fearful Jedi that lead to their own downfall. In short, The Acolyte should inform The Skywalker Saga.
“I mean, as you know, it’s set 100 years before the prequel movies, and it’s kind of an explanation of how the Sith infiltrated the Jedi. It’s a Sith-led story, which has never been done before. It’s been really fun to film! The cast is amazing, and the director and crew are just lovely. The whole experience is wonderful, and I’m already excited for people to see it.” --Dafne Keen
To see the dark side difficult it is.
Palpatine is using several techniques such as force stealth.
Force stealth also referred to as Force Concealment or Buried Presence was a power that was used by highly skilled Force-sensitives to mask their Force alignment (Light or Dark), their ability to use the Force, or even their entire presence from other Force-sensitives.