If you are trying to stretch as much and as hard as you can but things are simply not improving chances are high there is 1 simple thing that is holding you back!

Lack of progressions!

In weight lifting these come easy. You would not attempt to train for a 200kg Deadlift by doing 200kg deadlifts. Instead you would start with an empty bar and load it over time. Unless already very advanced you can not expect to improve your middle split by practicing the actual Middle Split. Progressions do not come as easily in bodyweight training. We need to break things apart and go slow.

You are stretching but not at your level. If you do the wrong progressions for your current state of flexibility you can not expect things to move forward quickly. You will hold yourself back or maybe even get injured!

Growing up I always thought the best way to stretch is to push myself down as hard as possible trying to reach my goal position at any costs even if it is just for the break of a second. Turns out with this just like with many other things I could not have been more wrong. If you don’t spend at least 45 sec in a stretch your flexibility won’t improve.
Now if you are doing a progression that is too hard for you it will be impossible to stay there long enough to actually get a stretching experience.
Additionally your muscle needs to relax in order to lengthen. If you are in loads of pain there is no way your muscle will let loose. Instead the muscle will engage fighting against exactly what you are trying to do. 1 of 2 things can happen now. You either push long enough until the muscle gets too tired to resist and you slide down way further than you should be at this point causing extreme soreness and injuries or the muscle will hold strong but the tendon, the place where the muscle is attached to the joint will give way and rip. Either way. We want to avoid this at all costs.
Take away: Stretch easy. Find a progression where you have slight discomfort and stay there for at least 40sec.

If the drill is not right for you there is no chance for you to actually go all the way to your end range of flexibility. You will be stretching somewhere between your current potential and a big void of nothingness. In order to improve flexibility you have to stretch at your own personal end range of flexibility. Anywhere before that might aid you to get rid of muscle soreness and help on your path to post workout recovery but you won’t actually increase your flexibility.

This one goes without saying. If your form is bad and you apply pressure you will get injured. The first problem point is of course the stretched area. If you push in an angle the stretch doesn’t happen as planned and you can pull your muscle or injure the joint. This does not happen as often though because if you are stretching your hamstring for example you are paying close attention to the hamstring and chances are high you won’t hurt it.

The bigger issue is the opposite side of the body that supports the stretch where your mind is not currently at. Sticking to the example of the hamstring in the Runners Stretch. It is tempting to round the back to get the forehead down towards the floor. This takes all load out of the hamstring and puts it into the rounding of the lower back. Just demonstrating this mistake for photos or videos gives me a bit of a sore back. Here it is vital to start with a straight or even slightly arched lower back. Flip your hips into an anterior pelvic tilt and only go as low as possible without rounding the back. Keep your form on point to stretch what you are trying to stretch and keep everything nice and healthy.