Hello ,
try yo add the keywords
encapsulation replicate
For local SPAN, if the encapsulation replicate keywords are specified for the destination port, these packets appear with the original encapsulation (untagged, ISL, or 802.1Q). If these keywords are not specified, packets appear in the untagged format. Therefore, the output of a local SPAN session with encapsulation replicate enabled can contain a mixture of untagged, 802.1Q, or ISL tagged packets
see
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_25_sea/configuration/guide/swspan.html#wp1200733
Hope to help
Giuseppe