Try what R Gould has suggested then there is no confusion on times for each section. Unless confusion has crept in I can see no other way in which you can have the problem you describe, given that you are sure that all other things are identical between the test strip and full print.
With some beginning printers the constant test strip interval method does produce confusion because the strips becomes harder to discern at longer exposures, and sometimes the first exposure doesn't show with an arithmetic test strip (e.g., 5+5+5+5...). At least that has been my experience working with introductory-level students in a community college darkroom. It is also very common for them to confuse the darkest strip with the least exposure and vice versa.