How can some Orthodox conversions to Judaism be valid but some are not? They are the same stream of Judaism?

There are some reliable bet-dins, and there aome less reliable ones who border on the conservetive.

Here is a list of recommended ones from the website on halakhic conversion (conversion according to Jewish law):

http://www.halakhicconversion.org/recommended_bet_...

Almost all Orthodox rabbis will accept a conversion if it came from another Orthodox rabbi. There are, however, serious differences amongst Orthodox and one may not accept a ruling from another.