Workflow Steps are not executing after one another

Hi, 

Would anyone be able to confirm me that if I created different workflow steps, they should be executing in order? 

I created a workflow to create a specified size of thumbnails

Then use that thumbnail in a custom process to create a folio manually. 

Another custom process is to call an API int he dps publisher. 

I notice that sometimes, my thumbnail are not generated and the custom process is already started to create the folio manually already.  Therefore, my folio ended up with no cover images. 

I also put in some loggings that the third step has also been triggered way before any of the first and second steps are finished. 

Are workflow steps running in parallel?  Did I do something wrong to make this happen like this?  

I've attached a picture of my workflow.