The Sandisk messes up my audio albums

In WMP i have my albums neatly organized: album art, name and songs - even old albums that I coudn’t find on the internet.

I led WMP synchonize with the Sandisk.

All albums and songs appear in Sandisk, but messed up:

Album art is applied for only very few albums, sampler albums appear multiple times (I guess as many times as there are different artists), some other (older?) albums are split for unknown reasons.

It all became rather unusable.

Is there a solution?