There several threads that go over this issue. KISS, I used a simple system that has passed audits. Over documentation is essentially a waste. Train, train, train.... We had some instances where things were not being done properly causing some extra work, so it was decided that we needed a WI on this and as you would guess it did not help.
Okay, please keep in mind as you read my response to this...I fully recognize that every organization is difficult and what works for one company will not always work for another....
First off, deciding that it was the lack of a WI was a root cause analysis that didn't dig deep enough. But please don't blame the WI for not solving the problem.
Secondly, training is all well and good - I'm a firm believer in the benefits of On-The-Job training - however, what happens when people start to retire? How do you help to continue a consistently performed process?
Somewhere on the Cove is the story about the monkeys in a cage with a bunch of bananas hanging down. As people leave, there should be some form of media for retaining the technical knowledge of the process. A documented process helps in this area.
A documented process is also great for companies who have activities such as standards/process auditing and job/task observations (i.e., safety focus). It provides a clear checklist to the auditor to ensure that all is performed as documented or, even better, the operator has a clear way of showing how a document should be modified to match the current process.
I'm not saying document everything. Far from it. But those processes which are key to your system and the organization's ability to consistent (and continually) meet Stakeholders' requirements, a documented process will greatly assist in the ability to retain knowledge about the process.