No, its catchrate is rather low as for most other legendaries. However if you fail to catch it you are able to try it again as Keldeo will re-appear in the same location again.
Unfortunetaly the game can have issues when using mobile data. I would suggest to use WIFI instead or try to use a VPN. Both options should fix the problem you are experiencing.
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Did you click the rock in the Wailord after beating the boss in there to get a Gem and did you talk to the Golem after beating the Mega-Charizard-X-Boss in the Mountain Maze?
There are 2 other parts of the quest you have to do. I would suggest to explore the event area a bit more and I am sure you will find the other parts of the quest.