Failure to start WCA process - gaps in fit notes

Just a bit confused by DWP’s stance on this one.

My client claimed UC in June 2019, reported health conditions and submitted fit notes. Now, my client is somewhat unreliable on account of her mental health, resulting in gaps in fit note provision since then. However, there have been multiple periods of 3 months+ covered by fit notes. So she should clearly have been called for a WCA at some point in the past. DWP have never imposed any work search requirements.

Due to attendance issues at our service, I’ve not been able to challenge the lack of a WCA at those times, and at the moment she hasn’t declared a fit note (although her GP has apparently provided her with one covering her from 2018 to now).

Having spoken to DWP today, they are refusing to start the WCA process - and saying they will start it once she provides/declares an updated fit note.
I’m progressing that now, but is DWP correct in this stance? Essentially that they can’t start the WCA process for a past period?
Even if she was healthy now, there is clearly a period in the past where she could have had LCW/RA.

I would make a complaint through the complaints resolution team. Ask why the WCA process was not started after the first sicknote was provided.
I am currently working on a project looking at when people win LCW appeals and when they are being back dated to. There are cases when UC are not back dating to 3 months after when the first sick note is provided which is what they should do.
Talking to UC gets you nowhere. I have found once a complaint is made you quickly get the info you need to see if UC has either misadvised or not done what they should be doing.
Regulation 28 of the UC regs 2013 states that once a fit note has been provided, a waiting period of 3 months applies, during which the claimant should be assessed. After this waiting period a decision should be made regarding capability for work.