it sounds like a buffer problem since the problem started when you made new buffer. Did you measure the pH of the buffers with a pH meter (that has been calibrated). Broad range pH paper is not very accurate but it does look as if the pH is too low.

Thank you very much for your answer Michael J. Benedik. I do not know how many times I prepared the transfer buffer. Last time I checked the pH of the buffers with pH meter (just before checking, I soaked the pH meter to solutions with known pH). There was a difference, however, I thought it could be due to temperature. If we assume pH meter is broken, I also saw that adjusting the pH is not recommended. Thus, I do not know what can I do for troubleshooting.

You could try measuring the pH of your buffers on a different pH meter in someone else's lab, just to double check.

You might also double check your recipes to make sure there wasn't some simple arithmetic error that crept in.

You might see if someone else in your department is also doing blots and borrow some buffers from them to rule out equipment or materials failure.