UDP Portscans Are Not Capture

["Grigoris Vidakis" <gvidakis () lab epmhs gr>]

Hi!
I run snort Version 1.8.3 (Build 88) in linux 7.2 (2.4.17) which alert me for the udp portscans correctly 
(portscan.log, snort.fast,snort.full) 
BUT when  i run snort Version 1.8.7 (Build 128) in linux 7.3 (2.4.18-3) with the same snort.conf and a snort binary 
file as the input (-r), captured from 1.8.3, which had  alerted me about udp portscans),  snort 1.8.7 does not alert 
the udp portscans!!!

Below is the snort.conf which i use for the 2 sensors.

var HOME_NET any
var EXTERNAL_NET any
var SMTP_SERVERS $HOME_NET
var HTTP_SERVERS $HOME_NET
var SQL_SERVERS $HOME_NET
var HTTP_PORTS any
preprocessor frag2
preprocessor stream4: detect_scans
preprocessor stream4_reassemble
preprocessor http_decode: 80 -unicode -cginull
preprocessor rpc_decode: 111
preprocessor telnet_decode
preprocessor portscan: $HOME_NET 4 3 portscan.log
output log_tcpdump: snort.log
output alert_full: snort_full
output alert_fast: snort_fast

does anyone have an idea about what is wrong??