Death of Zebra Finch

Question
I bought 2 Zebra Finches 3 months ago, 1 male 1 female.  They were doing good until a week ago, i found the female dead at the bottom of the cage first thing in the morning.  My local pet centre suggested i get 2 more females which i did only to find them both dead this morning.  I contacted the pet centre again and they were very unhelpful, telling me they will phone me back on Monday!  My male bird is still fit and well and is showing no negative symtoms.  He seems unaffected by what is happening, no behavioural changes.  All the birds were eating and drinking well and were very active, prior to their deaths they didn't appear to be unwell.  I am deeply worried especially as the pet centre has given me the brush off this morning.  Have i done something wrong? What could be causing the birds to die?  I consider them to be well cared for and have followed all the advice i have been given.  I don't think my male is overly aggressive however he is a very active and noisy bird compared to the females, he is also quite large, whether that has significance i do not know.  Please help.

Answer
It's hard to tell what happened and who's at fault here. I find that store bought birds are often ill and require immediate vet care after being purchased. I'm going to try and see what the problem could be by asking a few questions. Please answer them truthfully.
1. When you hold the bird up to your ear do you hear clicking? (If so, sac mites did them in)
2. Do you feed your birds with vitamin fortified food? (If not, start doing so)
3. Do you buy your bird food from well known pet stores? (If not, start)
4. Do you give your birds fresh water daily and wash the water dish a couple times a week with a tiny bit of bleach? (Bacteria builds up in water. Very important to keep it clean)
5. Do you clean your birds food dishes once every two weeks? (Even if there is still food in the dish, just throw it out. Again, bacteria)
6. Do you change the cage tray once a month? (again, bacteria)
7. Do you have a heater for your birds? (little known fact is that finches need heat. Have the cage around 20-25C and that will be fine. 28-30C if they're sick or molting)
8. How old are your birds? (This can be checked by looking at the band. Some pet stores keep older birds at the same price as the young ones and don't tell the buyer their age)
9. Was the bird in a cage with confirmed sick birds? (Really sick birds will sit in the corner on the bottom of the cage puffed out)
10. When your bird sleeps, does it tuck it's head into it's feathers? (A healthy bird will either tuck it's head under it's wing or into the feathers on it's back. A sick bird will just sleep as is)
11. Does your bird have a cuttle bone? (Not immediately life threatening but birds need calcium and the cuttle bone helps keep their beaks trimmed so they can eat easier)
12. Are you giving the birds any human food? (some human food is lethal to birds)
If you are unsure of the smaller details of caring for finches, please let me know. I will be happy to help you anyway I can.