Losses and/or inefficiencies using a DC/DC voltage regulator?
Your problem is most likely due to 1 weak cell in series.  When batteries are overstressed, they fail sooner with fewer than rated charge cycles.  The balanced voltage at the start-of-life becomes accelerated towards end of life as the weakest cell charges up fastest and drains fastest.

Unfortunately the quality of LiPo cells is not only measured in mAh, ESR but the degree of mismatch unless you have an internal active balancer. Passive balancers are ok up to a point because they are designed to a thermal limit which means a certain percentage of the cell charge rate power per cell. e.g. x% of 4Vx3A= 12W so they often rely on balancing only after the CC phase when currents are declining.


  You may want " max power or max current" which are different and not possible to get both as it is parameter dependent unless you do as Mfg specifies.


Max power out of a battery uses higher CV charge voltage ( e.g 4.2, 4.3) and lower cutout voltage at lower current rates, which leads to shortest life.

Whereas Max current and battery life, uses higher quality balanced batteries with less memory effect and lower ESR and may reduce CV to &lt;=4.1.
good brand: e.g. Enerloop 5th gen