The nature of the seals, and the summoning techniques themselves are probably different. It would seem that Madara, and later Obito, simply had summoning contracts with Kurama, and thus while they could summon it, they were could be restricted by outside forces (Madara was shown to be unable to summon Kurama from within another seal, and Obito's contract was shown to be cancelled by Minato.) Madara may have also experienced difficulties in summoning Kurama due to it being incomplete, and therefore the summoning technique to pull it out was not as potent as the eight trigrams seal keeping it in Naruto.
As for the Gedo Mazo, I believe Madara explained his rinnegan's awakening as granting him two techniques. One was to break the seal on it, and the other was to summon it. As I am certain that the six paths: chibaku tensei is stronger than the seal Obito used to seal the ten tails into himself, and Madara was able to break the chibaku tensei, it is understandable that Madara would be able to break the seal on Obito and then summon the Gedo Mazo afterwards (which is exactly what he did.)
What are your opinions on this?
