One bad test is not a signal
Everybody has a bad test. A single low score after a run of fine ones is almost always the week rather than the subject, and reacting to it teaches a student that one paper can decide something.
Three chapters is
Algebra compounds in a way most subjects do not, because chapter four assumes chapter three cold rather than roughly. Three chapters of not quite following is the point where the next one stops being learnable at all.
Watch the clock, not the grade book
The earliest tell is time. A student who used to finish homework in forty minutes and now takes ninety has already lost something; the grade follows about a month later, which is a month you did not have to spend.
When to wait
If it started with a teacher change, a move or a bad term at home, give it three weeks. Some of it is not the maths, and tutoring a student through something that is not the maths does not work.