Why didn't my egg fertilise after ICSI ?

We had our 1st cycle in July this year with ICSI as I had just turned 45 years and my partner is same age, we had a very good grade 1 embryo put back but didn't implant.

So we have just now had another cycle on the 24th November this month, I had a mature egg follicle size was 19 mm sperm was good they done ICSI again but this time they said it didn't fertilize ?

They told us the egg was good and the sperm was good but it just didn't happen.

How can I have a good egg and sperm but didn't fertilize using ICSI . I could understand it if the egg wasn't a good one or sperm, and how come we had a grade one embryo few months ago and now nothing?

Does this sound right to you ? ICSI with a good egg good sperm but no fertilization I cant get my head around I've been so upset.