Why were we told in the past the mRNA vaccines weren't effective against the variants, but now we are being told they are?

They learn more as more people get injected.

Please provide some evidence where doctors or scientists said that the vaccines would not be effective against the variants.

What they initially said was they did not know if they would be effective because they had no data. As data became available they were able to tell us they were effective against current known variants.

Science works on data. Without information you cannot say anything. Once the data is available you can determine what is happening.

Alternatively call the data evidence, which is what it is. As hcbiochem wrote that is the stuff science works with.

You were initially told that the vaccines "might not" be effective against the vaccines, but evidence (you know, the stuff that science works with) has shown that they are.

maybe theyve found out that they are

Answer to your question is this question "Why people need flu shots every year ?"

They tweaked the vaccines.