subprocess.popen returning empty string - json

Question

There was an earlier question on this, but the asker was just overwriting their output and solved their own problem.
I'm using a subprocess.popen to read video information and write the output to a json. It works fine on MOST videos, but on others is returning an empty string on others - even though it runs fine from the command line. I tried it several times and am getting the data fine through the command line.
Here's the relevant part of the script:
out_prj.write('[')
for m, i in enumerate(files):
print i
out_prj.write('{"$type":"BatchProcessor.Job, BatchProcessor","Id":0,"Ver":1.02,"CurrentTask":0,"IsSelected":true,"TaskList":[')
f_name = os.path.basename(i[0])
f_json = out_folder + os.sep + "06_Output" + os.sep + os.path.basename(i[0]).split(".")[0] + ".json"
trans_f = out_folder + os.sep + "04_Video" + os.sep + os.path.basename(i[0]).split(".")[0] + "-tr.ts"
trans_f_out = out_folder + os.sep + "06_Output" + os.sep + os.path.basename(i[0]).split(".")[0] + "-tr-out.ts"
ffprobe = 'ffprobe.exe'
command = [ffprobe, '-v', 'quiet', '-print_format', 'json', '-show_format', '-show_streams', i[0]]
p = sp.Popen(command, stdout=sp.PIPE, stderr=sp.PIPE, shell=True)
out, err = p.communicate()
io = cStringIO.StringIO(out)
info = json.load(io)
print info
filea = open(f_json, 'w')
filea.write(json.dumps(info))
filea.close()
f = open(f_json)
b = json.load(f)
print b
#########################
###################
f_format = str(b['streams'][0]['codec_long_name'])

Answer 1

Your code ignores error messages (err variable). print err or don't redirect stderr to see them.
Unrelated: the json handling in your code is insane: most operations are redundant.
To save output of the subprocess to a file:
import os
from subprocess import check_call
f_json = os.path.join(out_folder, "06_Output",
os.path.splitext(f_name)[0] + ".json")
with open(f_json, 'wb', 0) as file:
check_call(command, stdout=file)
Note: shell=True is not necessary here. If subprocess can't find ffprobe.exe then specify the full path e.g. (use the path appropriate for your system):
ffprobe = r'C:\Program Files\Real\RealPlayer\RPDS\Tools\ffmpeg\ffprobe.exe'
Note: r'' -- a raw string literal is used to avoid doubling the backslashes.