Split body on imported STP files

I have imported STP files sent to me by a friend who works in Moment of Inspiration. The first file I was able to use the split body function, cut a add to the part and the rejoin using the union function. However now he has sent me some more files same format etc. now when I use the split body function along a plane it splits the drawing into two parts, now without doing anything at all to either part I try and union them again, I get the message Boolean has failed.
Can any one give me any ideas as what it was working but now it is not?

Hello @Ali3d,

Please open a support ticket and send us the file before splitting and the split one too.
https://support.shapr3d.com/hc/en-us/requests/new

Thank you Peter, I am checking with my friend as there seems some of his files are corrupted, but others are ok, I will check before I send them through.