OpenSAML user discussion

Hi am facing a similar problem using SAML and XML signature.
My scenario is little differrent. First an unsigned saml assertions are 
inserted to the SOAP envelope. Later through a request handler the whole 
envelope is signed useing RSA algorithm. At the receiving end while tried to 
verify it, the original digest and the recalculated digests are getting 
differrent. So the signature verification is getting failed. But if the 
original soap message doesn't contain the saml assertions verification is 
passing.. Do we have to use any special canonicalization method while signing?
or the the saml assertions has to be signed alone? 

Thanks 
Deepak