Greetings @Oshenlynx,
You’ve already got some fantastic advice here about possible reasons as well as ways to go about fixing the situation 
The only other suggestion I can think of is that, perhaps while you are casting the spell, are you are worrying too much about bad outcomes? Worrying about spell outcomes while casting can become a self-fulfilling prophecy.
While it is natural to be hesitant (especially when beginning!) if you let your mind go beyond wondering and start to fixate on possible bad outcomes, even visualizing what could happen if the spell goes wrong, or strongly doubt yourself during the spell- then it is very likely those will happen/the spell will fail. Magick is a manifestation of our intentions- what you are focusing on is what you are casting.
As someone with anxiety, I know it can be really tough to calm the mind and set aside stress, worries, and fear. To overcome this, I’ve found that doing some peaceful grounding exercises and meditation before beginning can really help to collect focus in a positive way and gather self confidence 
The Meditation Roots Course has a great introduction lesson to Grounding- I recommend taking a listen and giving it a try! 
I would interpret a flame burning bright and strong as a good sign, and the Flickering Magick Pyromanyc Guide says something similar:
I don’t think you need to repeat the spell! 
Wishing you all the best, @Oshenlynx- Blessed be! 