Results of an experiment are statistically significant during 2 weeks and are not during 2 other weeks

I am analyzing an experiment where test variation had statistically significant result for some outcomes during 2nd and 3rd weeks with a difference of 20-30%, but had insignificant results for other 2 weeks (1st and 4th) with a difference of ~0%.
How can I address interpretation of these observations?
What the appropriate statistical method to choose to check if the difference in observations among 2nd+3rd and 1st+4th are just within random variance?
Can we still make a valid conclusion for this experiment based on total results?

We have checked any external factors which could influence the results and couldn’t find any.