content missing when a file is written into another file Python

Question

Im an entry level Python developer working on a requirement where I need to merge so many text files in forder A into one file in Folder B. I wrote the below code which is working but its not writing the entire content from files in folder A.

 import os
 a = open(r"C:\temp\datsOutput\New folder\output.txt", "w")
 path = r'C:\temp\dats'
 for filename in os.listdir(path):
     fullPath = path+"\\"+filename
     with open(fullPath,'r',encoding="utf_8_sig", errors="ignore") as ins:
            for line in ins:
                 a.write(line)

the around 6000 lines from the below line are missing.

 ROUT:=  996.6mm
         ANGL:=  95degree
         ORRF:=  =0/0
       TMRREF:=  =0/0
       REPCOU:=  0
    END
    NEW CYLINDER 26 of SUBEQUIPMENT /FA-3101/MAIN_M1_DAVIT
           NAME:=  =805324448/24319
           TYPE:=  CYLI
           LOCK:=  false
          OWNER:=  /FA-3101/MAIN_M1_DAVIT
           PURP:=  unset
            POS:=  W 537.345mm S 46.083mm U 0mm
            ORI:=  Y is S and Z is U
           LEVE:=  0 10
           OBST:=  2
           DIAM:=  76.2mm
           HEIG:=  304.8mm
           ORRF:=  =0/0
         TMRREF:=  =0/0
         REPCOU:=  0
       :PSTATUS:=  unset
    END
    NEW CYLINDER 27 of SUBEQUIPMENT /FA-3101/MAIN_M1_DAVIT
           NAME:=  =805324448/24320
           TYPE:=  CYLI
           LOCK:=  false
          OWNER:=  /FA-3101/MAIN_M1_DAVIT
           PURP:=  unset
            POS:=  W 537.345mm S 46.083mm U 6.125mm
            ORI:=  Y is S and Z is U
           LEVE:=  0 10
           OBST:=  2

for testing purpose the folder A now has only one file which has 204034 lines and the script has written 203799 lines, is it a problem of encoding? I simply cant go any direction from here any small help is greatly appreciated and saves my job, thank you