dynamics crm - Why does my CRM workflow steps run individually but not when two steps exist? -

I have created a custom workflow activity that copies an attachment from an email to an email, both the lookup parameters I workflow Set up the assembly, made a case with an attachment and an email I then used workflow design to create a custom workflow that runs my custom activity Attachments were copied well

The first use of the custom workflow assembly is to create an email before copying the attachment so I created a new workflow that created an email using details from a case. I set it as a manual workflow because what the end user is doing (CRM advisor). I attended that workflow and was made as expected of an email.

Then I modified the second workflow to add the second step. The second step copies the attachment to the email attached to the current case. When I started the workflow, it failed on stage 1.

I modified the workflow so that step 1 can not be used in step 2. Instead, I used an existing email according to my first test. This means that when two steps are performed individually and if they move together they should work because there is no link between them. However, when I ran the workflow, it failed on stage 1.

Can someone suggest that why is this happening?

I have received the answer so I have posted it here. I had a class that was not used with [serializable] attribute once the feature was added, the problem went away.