I can sort of reproduce your problem if I use the following code. The problem may be, that you are not opening the table read-write "fopen(cFileName,2)".

*&
*& Create the file!
STRTOFILE("0001","c:\test.txt")
CVALUE=FILETOSTR('C:\test.TXT')
? CVALUE
*&
*& Open the file and update!
*& use this one to open read-only
lnHandle = FOPEN("c:\test.txt")  *& FOPEN("c:\test.txt",2)

FPUTS(lnHandle,"0002")
FCLOSE(lnHandle)
*&
*& The update did not happen, becuase the file was opened read-only!
CVALUE=FILETOSTR('C:\test.TXT')
? CVALUE