Timur was indeed a good general. However, many of his enemies presented him with opportunties that he took. For example, the Khan of the Golden (Kipchaq) Horde actually met Timur in open battle as opposed to allowing the steppe to wear down Timur's army, which despite it's heavily nomadic influence, couldn't have survived out in the open steppe indefinetly.
And it seems that unlike the Mongols, he never really took long enough to consolidate his conquests, causing him come back and fight in the same spot multiple times.