For six neutron star atoll sources spanning wide range of frequencies we investigate whether the relationship between the rms amplitudes and quality factors of the observed kHzQPO LU display features that could have a significant meaning in terms of theproposed QPO models.We find for all the six sources that after the twin kHz QPOs pass a point (or the narrow interval) where their ratio R equals 1:5 the lower/upper oscillation becomes stronger/weakerthan other one with increasing QPO frequency. Existence of a similar effect close to R D 1:33 or R D 1:25 is also indicated. This interval lies between frequencies corresponding to 3=2 and 4=3 (or 5=4) frequency ratio. This finding implies restrictions to the orbital QPO models (both hot spot- and disc oscillations- like) and also to QPO modulation mechanism. In a wider context, our results may indicate the existence of an energy over flow between the upper and lower QPO mode when their ratio is close to ratio of small integral numbers.