When I read your question, my initial thought was you were working multiple shifts and that the same task was being worked upon by multiple people - some developer working during the day and some at night - sort of a disjointed paired-programming :)
But it looks like that's not the case. Each person is working on their own tasks - and they just happen to be working at different times.
If a person is working on a specific stage of your workflow - and has the capacity to work - but is unable to because of the WIP Limit on the column, clearly there is a mismatch between the WIP Limit for that column and the available capacity for that column.
The great Don Reinertsen has said in one of the Lean Kanban conferences that a recommended WIP Limit for teams new to Kanban can be twice the average amount of work being done at any time in a column (more about that here). So, I am not sure you have a high enough WIP Limit and this issue is an indicator of that.
How do you decide what WIP Limits to define? The following are some of the guidelines -
- Twice the average WIP as mentioned above as "prescribed" by Don Reinertsen.
- Keep a buffer for any blockers - cards that are blocked by team members due to their inability to continue working on them due to some dependency
- Number of tasks a team member is working on at a time and the time it takes for them to do any one task. For example, a dev team member will typically work with 1 task at a time, while a marketing team member might have 2-3 smaller tasks that they might be expected to complete in a day
- Unexpected tasks that need a person to put something on hold and work on the interrupt
A general thumb-rule I have seen most teams use is a WIP Limit of 1.5 times the number of people assigned to work in a specific column.
Ultimately, your WIP Limits must be defined based on your own context. If you have people waiting to take up work but can't due to a low WIP Limit, you have an over-capacity issue. If you have too many tasks in a column not being worked upon, you have a low capacity/ high demand or multi-tasking related stagnation of work (people not being able to complete any one task because they have too many balls in the air).
I'd say, start by increasing your WIP Limit to at least 1.25 times the # of people in and then see how the system behaves.