Unable to delete reference document using API

I am using below code to delete a document reference from Part.

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String strLinkId = new StringBuilder().append(WTIntrospector.getLinkInfo(WTPartReferenceLink.class).isRoleA("references") ? "roleAObjectRef" : "roleBObjectRef").append(".").append("key").toString();

  QuerySpec queryLinkDetails = new QuerySpec(WTDocumentMaster.class, WTPartReferenceLink.class);

  queryLinkDetails.appendWhere(new SearchCondition(WTPartReferenceLink.class, (String)strLinkId, "=", PersistenceHelper.getObjectIdentifier((Persistable)doc.getMaster())), 1, 1);

  QueryResult queryLinkDetailsResult = PersistenceHelper.manager.navigate(part, "references", (QuerySpec)queryLinkDetails,false);

  if (queryLinkDetailsResult.size() > 0){

  while(queryLinkDetailsResult.hasMoreElements()){

  WTPartReferenceLink wtpartlink = (WTPartReferenceLink)queryLinkDetailsResult.nextElement();

  parVersion = part.getVersionIdentifier().getValue();

  parIteration = part.getIterationIdentifier().getValue();

  WTPart copart = (WTPart)com.ptc.windchill.enterprise.wip.WIPUtils.getCheckOutObject(part);

  if (!WorkInProgressHelper.isWorkingCopy(copart)) {

  copart = (WTPart)WorkInProgressHelper.service.workingCopyOf(copart);

  Persistable localPersistable = PersistenceHelper.manager.delete(wtpartlink);

  }

  part = (WTPart)com.ptc.windchill.enterprise.wip.WIPUtils.getCheckInObject(copart,"Check In Comments");

  parVersion = part.getVersionIdentifier().getValue();

  parIteration = part.getIterationIdentifier().getValue();

  }

  }

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But some how it doesn't seem to work. But i see part is getting checked out and checked in.

Any help is greatly appreciated.