Hi SuperMaria
The sperm does have an influence after day 3, and a sperm issue can result in fewer blasts. However, sadly older eggs also have a part to play after day 3. With my OE cycles, I always produced a good amount of eggs but we only ever got early blasts and morulas by day 5 - slightly less than 50% of eggs collected. Our consultant told us that younger eggs can "repair" some of the damage in weaker sperm. Certainly our DE cycle supported this - we got top quality blasts on day 5 and it was 50% of the total number of eggs collected. The eggs were from a 24 yr old donor and were clearly much better quality than my older 40ish yr old eggs.
Good luck, I hope you find some answers.