This is not unusual. With our injection method we routinely get 50-60%+
survival and the spacing of the embryos seems to be important as you
have found out. We can usual get 60-70 embryos on a 22x40mm coverslip
(ie lengthways) for each round of injection. the easy way to see if your
embryos are spaced correctly is that there should be 2 embryo widths
between each embryo. i.e.something like this
O O O O O O etc
I have also found that the dessication time is important (we do 6 mins
in silica gel) - longer and the embryos are worse for survival.
There are also other important things - amount of DNA injected (a smidge
for us is best), host strain, phase of the moon etc.
If you want to know details my email should be on here somewhere.
I'll put my address here anyway
rjh at mrc-lmb.cam.ac.uk