Parallel 18650 battery pack voltage not stepping-up with Boost converter. Why?
I would have to assume that you are using some sort of XL6009-based converter module that has all the required passives. I have no information about the other module you are using.

A quick look at the XL6009 data sheet shows that the minimum operating voltage is 5V. Although there would be a bit of design wiggle room, there is no expectation that it would operate below that.

There are other options that might work with your application.