Timed sync stops working

[peasantmk2]

I use Syncml Pro on an HTC Desire Z, syncing contacts and calendar with memotoo. Over the past few weeks, the timed sync will work for an hour or so, and then stop. I have it set to sync every 30 minutes. The last log entry is the last sync that worked. Settings are as follows:
Timed sync: On
Show last sync time: On
Time Interval: 30 minutes
Wifi mode: Only if active

My last succesful sync was 7.00, and my wifi has been available all evening. Any suggestions as to what may be happening much appreciated thanks.