With embryos of grade 1 and grade 2 , if an ivf cycle fails , what could be the possible causes?

Grade 1 isn't 100%: Grade 1/2 embryos are subject to laws of chronological age, iow embryos from 40 year old eggs still have a very limited potential statistically for live births. Even in egg-donor cases its impossible to 'guarantee' embryos that succeed; estimates are around 1-in-4 embryos on d3 will become clinical pregnancies. Implantation failure is also an area that is poorly understood at this time, .

Complicated: First of all grade is only one part of embryo quality. The rate of growth and expansion of the embryo are most important. I transfer almost always a blastocyst embryo. Many grade 1 embryos early on do not look good later. Also pregnancy rates from grade 1 embryos are twice as high as grade 2 on average. Sthe uterus makes a big difference so proper evaluation is very important.