BQ51013B: power transfer possible with up to 12mm distance RX to TX, but initial start only possible at clase distance

I am using a bq51013B receiver with a pretty low Ls of 7.5uH C1 with 240nF and C2 anout 3.3nF. Q is about 130 (Rdc =0.34Ohms). Power transfer with 5W is possible once the initial srat is esablished (EVM module as TX). I can increase the distance between the coils under full load upt to 12mm but if the transfer is interrupted and/or shall be started at coil distance above 4mm it does not start. Rectifier voltage only reaches about 2.4V. Changes of C1 and C2 did not show any effect. My rectifier capacitor is 10µF, previously tired 22µF and feared that the charging time is too long but did not change,... I tougth maybe the communication modululation is not working well and changed the initial 22nF to 47nF no improvement. Is there anything else I can do to get the stuff started as whide spacings between the coils? I currently use a Würth 760308103206 coil since this is the form factor I can fit in our design. Did not find equivalent coil in the same form factor. Alternatives with higher inductance are of course welcom if available.