Sudden drop in production

In one of the layer farm, production dropped down to 40% suddenly, so there must have been suffering some major problem or the signal of facing infectious disease:

History of the flock:

1) Breed:Lohman White
2) Age:- 31 weeks
3)Feed consumed:-100kgs
4)No of birds:-- 1015
5)Body weight :- under weight as recommended by hatchery
6)De-worming--06/03/15
7)ND killed vaccine used:-13/01/2015
8)Lighting continue up-to 10 o'clock

Medication history:

Used three times,one after another
a) Enrofloxacin + Trimithoprim, after that
b) Erythromycin then c) Amoxycillin + colistin + ciprofloxacin

After using all of these then owner came to me & request me to visit his farm, I have visited the farm & found some abnormal fecal materials mucoid yellowish white,movement was normal, Less feed intake, I prescribed :

1) Pro-biotic
2)Liver tonic
3)Electrolyte
4)Bio-spark-v(ready-made protein+Energy+a.a+ micro-minerals

 I expecting more discussion , I am from Cox's Bazar, Bangladesh.