Hi and welcome,
knowing nothing about what you have and are using and how you are using it/them; I would suspect that the battery you have is simply not strong enough to provide the wattage you are requesting.
Please share with us:
Which RTA?
Are you using temperature control?
How many watts are you wanting?
How many watts do you get?
What is the build in the RTA? Single/dual/complex
coils/mesh
What is the resistance of the build?
What battery device are you using?
If it has removable batteries, what is the brand and model of the battery you are using?
Cheers
I