Idoc lost from SAP R/3 to XI

Hi experts,

I have the following problem:

sometimes an idoc fired from SAP (not through WE19) does not reach XI (I cannot see it in SXI_MONITOR). I've checked the following facts:

1. SAP R/3 status of the idoc is 03 (direction outbound).

2. SM58 produces no result.

3. IDX5 produces no result for that idoc.

4. 99,9% idocs of the same type fired in the same day work fine.

Resending manually the same Idoc using WE19 gives no problem and the idoc reaches XI.

any idea?

Thanks a lot,

Daniele