The first time Lucy casts Urano Metria she used her own power to defeat Angel with no problem. But the next time, when she uses it in The Grand Magic Games she has to combine her power with Gemini to cast it and says it was still weaker than the first time and since this was after her second origin was unlocked she would have at least 3 time as much magic energy (her origin would double it and gemini would double that) So what makes her first casting so strong.
The first time she casted Urano Metria, she received the spell from Hibiki of Blue Pegasus. He gave her one chance to cast the spell which she successfully did but the spell wasn't her's. Hibiki's power is "Archive" : he is able to transform a lot of things in datas that he can transfers to people. Thanks to this power, Hibiki has been able to give Lucy his knowledge about the spell and all his magic power (he gave her so much of his power that he fainted instantly right after).
After casting it, she couldn't remember anything and she had to learn the spell all by herself. The trick she found to overcome the difficulty of the spell was to combine her magic power with Gemini's, like Naruto mastered Rasengan thank to his clones.
Hope that helps
- Fairy Tail Manga: Chapter 144, Pages 14-20
- Fairy Tail Manga: Chapter 385, Pages 14-17