Pyshark / Tshark not going throught all packets

Question

I have a problem when going througt an openfpc pcap file(1gb) with pyshark. When i use (in console, as root):

lst = pyshark.FileCapture(TMP_NAME):
for p in lst:
    if(p.highest_layer=="FTP"):
        print "win"

It does not print anything, even if i am sure there are ftp packets in the file. If i go like this:

lst = pyshark.FileCapture(TMP_NAME, display_filter="ftp"):
for p in lst:
    if(p.highest_layer=="FTP"):
        print "win"

It prints "win". My questions are:

1) How can we explain this behaviour?

2) How can I see the ftp packets without filter?

EDIT: I have seen that this only happens when using .pcap files.The first code works flawlessly when using .cap files downloaded from the wireshark website. Which I find wierd since pyshark uses TShark, and should be able to read pcaps no problem.

Thanks