Basic problem with RBC lysis

I was wondering if anyone could give me some advice, i'm new to flow
cytometry and struggling with the basics.
my problem is with RBC lysis. i am using an ammonium chloride lysis solution

1X solution = 0.15M ammonium chloride, 10mM sodium bicarbonate and 1mM
disodium EDTA.
I make up a 10X stock for storage and make up the 1X stock on the day.
The blood is collected in EDTA BD vacutainers and i begin the lysis protocol
within an hour of taking the sample.
Practically all the protocols I have read lyse for 10mins so this was the
time course i planned to use. 
I used a ratio of 1:15,  blood:lysis solution. All done at RT.
The problem was that lysis was not occurring for over 20mins (i.e. i could
see that the solution had not turned translucent before this). I continued
with the protocol washing the WBC pellet twice with cold PBS and then
resuspending in 1ml PBS. 
When i did the cell count using 0.4% trypan blue (10ul cells : 90ul trypan
blue) the vast majority of the cells stained blue.
i presume so many of the cells are dead because they have spent so long in
the lysis solution and it has affected not only the RBCs but the WBCs too. 
so my question is why is it taking so long to lyse my cells? (these are
samples from healthy young people 18-24y)
and if anyone has there own protocol for ammonium chloride lysis or any
other cheap and cheerful lysis i'd really appreciate it.
thanks in advance
Sarah