You may need to stretch your hamstrings multiple times a day. Keep in mind that if you spend 5 minutes stretching your hamstrings per day, you probably spend 23 hours and 55 minutes in positions where the hamstrings are shortened and turned off (e.g., sitting, standing, sleeping). That means you're training short, weak hamstrings 99.7% of the day.

With that perspective, it's not surprising that progress is slow.

You may also need to do more contract/relax to strengthen the hamstrings in lengthened positions. This is often a big missing piece for people with stiff hamstrings.

This video playlist may be helpful for your hamstrings.