Pos on cheap tests, Neg on frer???

Has anyone else had this problem,

I took a early detection test. 10mlu.

It was a clear positive.

Took a frer - Neg.

Now I took a 25mlu test, and I can see the faintest line.

Took a frer - same sample Neg??

I’ just dnt seem to have any luck with these tests, or there nowhere near as sensitive as they say.

I read other woman having the same problems.