I was analyzing maleic and fumaric acids in biological extract using a Cogent Diamond Hydride column. After 10 perfect injections (%RSD of retention times around 0.5), I noticed that the peaks intensity diminished about 10 times, however the peaks still have the same retention time. What happened to the column?

Most likely, nothing happened to the column. Your mass spectrometer probably needs some attention. I would clean the ion source and check the nebulizer as a first place to look for issues.