I bet your batteries PCB is being tripped. The link to the HV atomizers do not give the actually resistance, but even with a 5.2Ω atomizer at 6v your max drain is 1.15a, with a 4.5Ω atomizer your max drain at 6v is 1.33a. The max drain on the blue Tenergy 900mah batteries is <0.9a.
The max drain rate on a battery is figured by taking the mAh rating times the C rating and then dividing by 1000. The C rating is assumed to be 1, unless it states otherwise, like ultrafires with a 1.5c rating and AW LiFePo4 with a 10c rating.
I would suggest gettting theses for 6v vaping:
AW LiFePO4 3 Volt Lithium Rechargeable Battery
Hope this helps.