They made a huge amount of progress.  

The ability to predict the positions of the planets was improved by more than a factor of 100, after 1500 years of no real progress following Ptolemy. 

However, 

They did not provide any simple underlying physical explanation for the behavior.

Kepler was even on the track of this breakthrough, but he was more concerned about keeping the planets moving and envisioned a force from the sun that swept them along a bit like a broom. This was a “special” force that only existed for this one purpose and hence it had no relation to a universal physical law.