We have had 2 silkie pullets since the beginning of July, the age was unknown when we bought them but they have not grown too much since we had them. However they are still not laying eggs. We dont feed them grower feed as they are mixed with the rest of the flock so just eat layer feed. Could this be why?
If you don't know the age, you really can't set a timetable for them to lay. When they start to lay is somewhat breed dependent but also can be very much different for each different individual. The Grower versus Layer is not a reason for them to not be laying. Both should have 16% protein and pretty much the same other nutrients. The only difference is in the amount of calcium. If I did not know the age but suspected they were too young to lay, I would feed all of them Grower with oyster shell on the side for those that need it to avoid potential problems with growing chicks eating too much calcium. It is correct that underfed chickens can be later in starting to lay, but there is no significant difference other than calcium in Grower and Layer. That is not your problem.
We also have 2 buff orpingtons pullets that we have had about 6 weeks and these are not laying either, however they still have not developed their comb and are just starting to turn red. The buffs also have a very unhealthy back end and water seems to pour out of them is this something to do with their diet?
Possibly, but probably not. As was mentioned, it could have something to do with worms. Check for mites and lice too, but I doubt that is the problem. You never know though, and mixing chickens like that, you never know what they give each other. Which brings me to a guess. It is possible that your existing flock has the protozoa that causes Cocci but they have developed an immunity to it. It is possible that the newcomers do not have that immunity and are suffering a bit from Cocci. Maybe not enough of a problem to cause them to go lethargic or be a serious threat to their life or a long term threat, but enough to loosen them up. After six weeks, they should be pretty close to getting their own immunity built up though. Cocci is not an automatic death sentence. If the area they stay in is pretty dry, they often are not affected by it all that much. Remember, this is only a guess, not a detailed diagnosis. And this is something that would not show up in a quarantine.
When you disrupt the pecking order by adding new chickens, you can disrupt laying until they get it sorted out. By six weeks, this should have long been settled, but I find that chickens are not really all that consistent. They don't all read the same behavior development manual.
Good luck!
is this something to do with their diet? 
