Flutter App that reads and writes to GSheets

I have a flutter app that has a google sign in, and reads and writes to a GSheet with a GScript. The problem I am having is with the google sign in and with the read function. The google sign in works fine the first time, but if I go back and try to sign in again, I am unable to do so. and in the read function, I am getting null back from the GScript even though there is data on the workbook. I need to have these problems fixed. Thanks in advance.