Low HCG. Help?!

I am currently 18 DPO, and 4 days late on my period. I’ve been getting positive tests for the past 5 days on FRER, dollar store tests, and Walmart brand tests. They are definitely BFP’s, but they are NOT getting any darker! Both the clear blue and first response digitals are still reading “no-“ and “not pregnant”, even with 8 positive tests otherwise. How bad of a sign in this that my levels are not rising? These tests are definitely positive, and I’m definitely late. But they’re lighter pink and haven’t changed over 5 days- so low that a digital still can’t read them. Will they rise, or is this likely to end in a miscarriage?