Poorly chicken (silkie) had to be put to sleep - cause?

Hi

Apologies if I ramble on a bit.  I have been on here before where one of our silkies (Goldie) was bullied and was poorly and not eating.  Took her to the vets and she had antibiotics and thankfully recovered - as a result of this we have bought a new hen house with a run for the three silkies and the 2 larger hens are now separated in another run. 

New hen house with run came Wednesday and the three silkies were put in this they all seemed happy - they have had a visit from the fox where they were all sqawking - but this has happened when they were in the other run.  One of the silkies (Juliet) I noticed was looking a bit sorry for herself on Friday.  I went off to work thought nothing of it.  Looked at her Saturday morning - she still looked sorry for herself - my husband then looked at her and we took her to the vets.  The vet said the kindest thing to do was to have her put to sleep - which we did.  She was just over 2 years old.  I noticed a soft egg a couple of weeks back one of the chickens had laid an egg with no shell - it was a bit squddy.  The silkies have not laid eggs for a couple of weeks now.  Goldie was in a separate cage as she was recovering from being bullied and in the last month has laid 2 eggs.  So this soft egg must have been laid by Juliet or the other black silkie.  Juliet was eating quite well so it has happened rather quickly.  We have bought flubenvet 1% just in case it was worms and cannot make head nor tail of instructions.  It has a scoop which holds 6g of wromer.  We have two separate feeders.  One for 2 large chickens (one is a rhode island red) and one for the 2 silkies.  Instructions say to do this over 7 days.  Do I just add one scoop for each feeder every day for 7 days?  Any ideas on cause of illness - how long do silkies live for?  Could it be change of habitat - the visit from the fox?

In a tub add 2kg of layers pellets, then add 1 level scoop of flubenvet 1%. You then pour these pellets into their feeders and just feed them on these medicated pellets for the next 7 days. If you run out of pellets then simply mix up some more. After 7 days go chuck any medicated pellets you have away and go back to plain pellets