Errore conversione da PDF a PDF/A-1a

We purchased a monthly subscription product , but we continue to have error when we try to turn a PDF into a pdf \ a -1a .

Here the error report:

Preflight - Summary Report for : Debit Note No. 143431_vp.pdf

Profile : Convert to PDF / A-1a ( prepared Pages 1 to 1 )

Processed by Carlotta.Andreoli , Date : 11/19/2014 24:32

corrections

Convert to PDF / A-1a ( 1 item )

Strength of sRGB color space fusion ( 1 item )

Create XMP metadata of the document compatible with PDF / A- 1 ( 1 item )

Downsampling / compression of color images ( 1 item )

Downsampling / compression of images in grayscale ( 1 item )

Downsampling / compression of monochrome images ( 1 item )

Reduce transparency single-level ( high resolution ) ( 1 item )

Remove all comments and form fields ( 1 item )

Remove all actions of sending , import and reset module ( 1 item )

Remove all actions JavaScript ( 1 item )

Remove all external cross references ( 1 item )

Remove all alternative images ( 1 item )

Remove embedded thumbnails ( 1 item )

Remove private data from other applications ( 1 item )

Remove document structure ( 1 item )

Remove contents hidden layer and reduce visible layers single-level ( 1 item )

Convert smooth lines in curves ( 1 item )

Detect And Merge Image Fragments ( 1 item )

Remove document structure compression ( 1 item )

Remove document structure compression ( 1 item )

Compress all objects not compressed with ZIP compression without loss of data ( 1 item )

Recompress LZW as ZIP ( 1 item )

Remove bookmarks invalid ( 1 item )

Remove invalid shortcuts ( 1 item )

Remove named destinations without references ( 1 item )

Optimize PDF for Fast Web View ( 1 item )

Fix font encoding ( CIDSet ) ( 3 items)

Insert CMap for CID fonts ( 4 items )

Remove unnecessary transparency groups ( 1 item )

Repair hierarchies bookmarks invalid ( 1 item )

Results ( Summary )

error

MarkInfo missing

PDF structured : Item Node of the tree missing

How can I resolve this errors?

Best regards