Early Pregnancy with Low hCG Levels: This is the most likely diagnosis given confirmed ovulation and a positive, albeit low, beta hCG level at 15 days post-ovulation (dpo). The absence of a period by 17 dpo further supports this possibility, as it suggests implantation may have occurred.

Pregnancy Loss or Miscarriage: A low beta hCG level that does not rise appropriately can indicate a non-viable pregnancy. The absence of a period could be due to hormonal changes associated with early pregnancy loss.
Ectopic Pregnancy: Although less common, an ectopic pregnancy could present with low and slowly rising hCG levels. The lack of a period and confirmed ovulation keep this possibility in the differential.
Hormonal Imbalance: Certain hormonal imbalances can affect menstrual cycles and ovulation, potentially leading to irregular periods and low hCG levels if pregnancy were to occur.

Ectopic Pregnancy: This is a critical diagnosis not to miss due to its potential for severe morbidity and mortality if not promptly treated. Symptoms might not always be present early on, making a high index of suspicion necessary.
Molar Pregnancy: Although rare, a molar pregnancy can present with low hCG levels initially and is crucial to diagnose early due to its potential complications, including the development of gestational trophoblastic neoplasia.

Pituitary or Ovarian Tumors: Rarely, tumors of the pituitary or ovaries can affect hormone production, leading to irregular menstrual cycles and potentially low hCG levels in the context of pregnancy.
Chromosomal Abnormalities in the Embryo: Certain chromosomal abnormalities can lead to failed pregnancies or very low hCG levels. While not directly diagnosed by hCG levels alone, they are a consideration in the context of recurrent pregnancy loss or failed pregnancies.