What Causes Spotting During Luteal Phase?

Hello! I am 31, my husband is 36. We have been ttc for over a year with no luck. All of my labs have come back normal as well as my husband's SA. My progesterone levels suggest a strong and healthy ovulation. We time intercourse well and are fairly stress free and healthy. My ultra sound showed no cysts, though I still need a HSG and a few other tests. I spot in my luteal phase a little bit and am worried my lining (measured at 10 mm before my period) could be an issue. Is spotting okay or can work against a pregnancy? Thank you! Lauren