yes it is definitely related with data input and data output. You need to make sure that the variables are mapped correctly in order to move the information from one task to the process and to the process to the second task.
In order to do that you need to go to the process designer click in your tasks and set the DataInputs, DataOutputs and Assignments.
DataInputs and DataOutputs just define the names of the variables that are going in and out from your tasks and the assignments define how the data will be copied from the process scope to the task scope and viceversa.
Take a look at the examples in the processes here: droolsjbpm/jbpm-playground · GitHub to get an idea about how these mappings work
Open the process that are in that repository and check their mappings.