The problem mostly arises due to the following causes
1. The Temperature of the Cement and fineness.
2. Performance of the Admixture.
3. Fines content and absorption levels of fine aggregates.
Its easy to control when you are producing concrete in-house by giving instructions to all Transit mixer driver and helpers because they are the only peoples who does addition of the water. when the concrete is outsourced, it becomes difficult to manage because and still its manageable with multiple control points.
Most of the issues can be controlled at Concrete Mix Design Stage only by understanding the clear requirement and expectations from the concrete mix.
Such as retention time - PCE gives the flat level of performance for a designed period of time whereas Naphthalene base can have gradual decrease in the performance with respect to time (Most neglected parameter while designing).
After designing of the concrete, the performance of the concrete admixture need to be verified for every new batch of admixture received and when there is conflict, should be rechecked with the controlled master sample to verify, either admixture is not performing or is there anything else impacting the performance.
The temperature of the cement leads to increase in the water demand as well as false setting type issues in the concrete, when you procure cement from direct manufacturer which comes in bulker directly from the cement factory, in many cases you will observe cement is very hot. this temperature leads to increased water demand + false setting.
Fines content and absorption of fine aggregate leads to increased water demand, increased absorption slurps the good amount of water from the mix and the increased surface area due to increased finer content also demands water for lubrication of the surface.
if above parameters are under control, the chances of not getting desired retention will be like once in a blue moon which can be made workable by re-dosing of admixture.