Niamh, I'm sorry about the result, and I'm glad you asked this here, because "within the normal range" is a phrase that deserves a range attached to it. General information follows; your own lab's figures and your own next step are for your clinic, who know your case.
The direct answer: two blastocysts from eleven eggs is an ordinary result, not a poor one, and the drop between day 3 and day 5 is where the biggest loss is expected. The arithmetic runs roughly like this. Of the eggs collected, typically 75 to 90 percent are mature; yours were 9 of 11, which is good. Of mature eggs, around 60 to 70 percent fertilise normally with ICSI; 6 of 9 is squarely in that band. And of normally fertilised eggs, roughly 30 to 50 percent reach the blastocyst stage by day 5 or 6, which from six gives an expected two or three. You got two. Louise's three cycles of 2, 1 and 3 from similar starting numbers are the same arithmetic with the ordinary variation on top; Mrs Okafor's four from twenty-one shows that a large collection does not scale the way people hope, and the cycle with none at all is the lower tail of the same distribution, more likely with age and fewer eggs but possible at any age.
On why "all six looking good" on day 3 became two: the embryo runs on the egg's stored machinery for the first two to three days, and its own genes only switch on at around the 8-cell stage, on about day 3. Embryos that look fine on day 3 and then stall are usually ones whose own genome could not sustain development once it took over, most often because of a chromosomal error carried from the egg or, less commonly, from the sperm. That is also the answer to whether a day 3 transfer would have saved them: it would not. An embryo that arrests in the dish on day 4 arrests in the womb on day 4. Transferring on day 3 does not add embryos, it removes information about which ones were viable, which is why most labs grow to blastocyst whenever there are enough embryos to justify it, and consider a day 3 transfer mainly when there are only one or two, to avoid reaching day 5 with nothing.
Eggs or sperm: the two stages point in different directions, loosely. A low fertilisation rate with ICSI points at sperm or egg activation problems; you had a normal one. Loss between day 3 and day 5 is more often egg-related, because the egg contributes the bulk of the chromosomal machinery, though severe sperm DNA damage can also show up as arrest after day 3. Mild male factor with a normal ICSI fertilisation rate would not, on its own, be my first suspect. At 33, a blastocyst rate of a third from normally fertilised eggs is unremarkable.
When the lab is the problem: the way to know is not one patient's numbers but the lab's. Embryology labs track their blastocyst formation rate per normally fertilised egg as a key performance indicator, and the international consensus benchmark most labs measure themselves against is around 40 percent, with anything much lower across many patients being a flag. You are entitled to ask your clinic what their figure is for your age group and how it compares with that benchmark, and a good lab will tell you. What a single cycle's two from six cannot tell you is whether the lab underperformed, because it sits comfortably inside the expected range. The site's guide to embryo grading sets out the whole week, the attrition at each step, and what the day 5 grades your two were given actually mean, which may help with the decision about the one that's left.
That decision, transfer the remaining blastocyst or collect again first, is a real one with reasonable arguments on both sides, and it depends on things I don't know: its grade, your reserve, your finances, and how you would feel with nothing in the freezer. Take the arithmetic above into that conversation and ask them to show you where your cycle sat in their own numbers. It is a fair question, and it is not an accusation.
