Post Stretching BPFSL as a newbie

Hi, apologies if this has been posted, I did not find a match via the search function.

As a newbie doing the Newbie routine at around 15-20 mins of manual stretches, I wanted to know if I need to be seeing a slightly longer BPFSL at the end of my workout? I’ve started incorporating fulcrums and at about 20 mins I feel I’m reaching my limit (so I stop there) and I feel slight achiness. I feel this reflects I worked it enough, but my BPFSL is unchanged. Pre workout vs post workout.

I’d like some advice as I don’t want to waste time with my stretches if no post workout BPFSL = no gains.
(I’ve reached 3 months of newbie routine without much change, so I’m trying to figure out why and change things up a little bit)