Cheapie line progression

I’m pretty anxious because I’ve had spotting for the past week. It’s pretty much resolved at this point. I’m not sure when I ovulated but I’m guessing I’m 16-18dpo based on when we bd’d. I’ve been trying to keep my mind at ease by watching line progression however the amazon cheapies I’m using seem to not be darkening anymore. The bottom one is from this morning 9/7....Is this a common issue with the cheapies or should they be darker at this point?