You are loosing your persistence file. (Or, it was never created -properly)
When you boot into the system the filesystem looks like
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If you try to add up the sizes you'll see it's bigger on the inside than on the outside. The closest to reality is
/dev/sdb1 at 7.5GB.
What it looks like from the outside, using another system to look at the LiveCD
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Personal data (your stuff) can be accessed though here.
If you have not put data in these directories (including
VIDEOS which was hidding when I made the graphic) , you cannot get it out of the system unless you boot the system. In these directories data is available, whenever, where ever, unless it's on fire. You can place data here when you're not running Navigatrix, and the data will be there after you boot the system.
It's this smoke and mirrors that makes it so versatile and still be near bulletproof.
But smoke and mirrors cause problem as there is only so much room on a 8GB stick. First, they're 7 point something GB and then when you load in a system and skeletal filesystem, a persistence file, a few charts, GRIBs, etc., there's not much room left.
I'm speculating you are loading, or have already loaded 2 tonnes of fertiliser in a 1 tonne truck. When you created the file, there is/was not enough space for the creation of the 2GB persistence file. The system acts like it's creating the file, but it lies.
Depending how you are budgeting your space, and whatever is taking up the space, you might try a 1GB persistence file to squeeze in.
...or install all your system file and store your data on the laptop's hard drive. It's your choice.
Regardless, a full USB stick will start to behave badly.