Packet loss during NAT on a CCR1036?

Hi,

I did a capture of a RTP stream on a CCR1036 because there were transmission problems. The main problem is identified as problem of encoder and decoder.
But I also noticed some strange packet loss during NAT. I did a dump file by packet sniffer tool and analyzed with Wireshark.
NAT rule is like this:
/ip firewall nat add action=src-nat chain=srcnat src-address=1.1.1.192/30 to-addresses=2.2.2.19
RTP stream is from 1.1.1.192 to 3.3.3.253

Left side of the graphics is the RTP stream analysis of the unnatted packets, right side are the natted packets.

Is this a real problem of the NAT or is it more probable that it is a problem of the packet sniffer?

Regards
Martin