Firstly, it is often harder than people realise to determine if progress is being made, and commonly an objective eye is needed to help point out elements where improvements are visible.
When progress is negligible or less than expected, there can be a multitude of reasons. Prior training history and current training volume obviously play a significant role, with genetics playing the next biggest determinant. Where genetics is a major factor, the term ‘non-responders’ has traditionally been used. However, this has been show to be an inaccurate (and not very helpful) term.
From studies, one would estimate that for untrained individuals maximal aerobic fitness over 6 months might on average improve by about 20%. However, this does have a very large range of 0 (zero) to 40%. For already trained individuals, you are looking more in the single digit range, with 5% often talked about as being an upper value.
Furthermore the studies that these calculations are drawn from incorporate training programs exclusively consisting of 4 or more training sessions per week. So if you are running only 2 or 3 times a week the rate of improvement is no doubt expected to be less. Consequently, this is normally the first aspect to consider if wishing to try and make quicker gains.
To discuss your personal situation, please feel free to ask in-app.