Inconsistent filament size?

Hi, sometimes I have problems printing my item, I bought filaments from china and for this one, the first spool I bought (other than came with the kit), sometimes giving me this problem, more like the plastic are not deposited enough to have it bonded with the previous and other plastics.

These problem can be a disaster because it sometimes occur on 1 layer but not others, some objects I printed doesn't have this issues.

Can I say these are low quality filaments because it occurs to me sometimes?

Please refer to the picture for the issue, sorry for the poor quality, it is hard to make it clear
[imgur.com]

I've found that layer bonding problems occur either when you're under extruding or printing too cold. If it's under-extrusion, it should be visually obvious - the layer should look like it's got holes in it. If not, try upping your print temp by 5 degrees to see if it bonds better. To test, try printing two towers side by side, upping temp by 5 degrees each time, until you get a print that holds together well.

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I've found that layer bonding problems occur either when you're under extruding or printing too cold. If it's under-extrusion, it should be visually obvious - the layer should look like it's got holes in it. If not, try upping your print temp by 5 degrees to see if it bonds better. To test, try printing two towers side by side, upping temp by 5 degrees each time, until you get a print that holds together well.

The thing is, I somehow get these thing almost once in a while though, I did flow rate calibration and yeah, I forgot to mention the parameters when printing the back case;

layer .2mm
speed ~50-60mm
filament retract 1.5mm
temp 250'c
flow 75% (most accurate during calibration)

If you notice on the flat area, there are 2 part of areas that are quite smooth then after that it became stringy. I wonder if this is due to air bubble in the hot end or simply is the filament problem?

It is hard to tell. My max temp would be 250 before the teflon tube in the heatbreak starting to burn so I wouldn't go beyond 250 and I already set 255'c is the maxtemp safety cutoff in marlin