Dick Van Dyke didn't mesh with Carl Reiner's dialogue during the first season of The Dick Van Dyke Show
The show just didn't work at first.
It's common for a show to not hit its stride until the second (or third) season. Series very rarely arrive fully-formed, for many reasons. Maybe contracts are drawn up for the first few episodes, and the wrong actors have been cast. There are plenty of examples of cast shakeups between the first and second seasons. Sometimes, there's an issue with the format. The show isn't shot or presented in a way the audience can connect with. More often than not, though, there's an issue with the writing.
Luckily for producers everywhere, audiences tend to be very vocal about what they do and don't like. Critics are actually useful in this situation. It's up to the series creators to respond to the feedback or not.
For The Dick Van Dyke Show, a lot of the problems were clear from the beginning. First, there was the fact that the network only ever reluctantly supported the show. According to producer Sheldon Leonard's 1995 memoir And the Show Goes On: Broadway and Hollywood Adventures, it was CBS' decision to place The Dick Van Dyke Show in the unenviable 8 PM EST timeslot, meaning the show would air at 7:00 in the midwest. According to Leonard, it was "a lousy time spot for an adult comedy."
"We went on the air in October of 1961," wrote Leonard. "We didn't do too well. Something was wrong with the show. More specifically, something was wrong with Van Dyke. He didn't seem at ease with the dialogue. It took us a good part of the first season to identify the problem; then it took more time to correct it. Dick's substandard performance, combined with the poor time slot, made it not surprising that we had disappointing ratings."
The problem was a matter of regionalism. Carl Reiner, who wrote the show, is famously funny, but in a very specific way. The comedy legend was a Bronx native and wrote dialogue consistent with his upbringing. However, Dick Van Dyke wasn't from New York, he was from Indiana. He sounded incredibly unnatural trying to deliver lines as if he were a Jewish comedian from the Bronx.
"It sounded all wrong coming out of the mouth of an Indiana Baptist," said Leonard. "We fixed it."
The rest, of course, is show biz history, as The Dick Van Dyke Show would find acclaim in its second season, propelling it into the pop culture mainstream, and creating a legacy we still celebrate today.