Dark images in Manual mode with 2 Broncolor strobes

[alisadeghy]

Hi

I'm trying to shoot some products with my Canon 70D and 2 Broncolor strobes (firing in full power) in camera Manual mode.
These are my exposure settings:
Shutter speed 1/100 sec.
ISO-100
f/16
I use my internal camera flash to trigger strobes and they fire correctly.
But my photos get dark randomly in same setting!!! For example in these images I use the exact same setting but only in different distances from product!

The exposure level indicator shows a small arrow downside -3 mark.
The other problem is that camera does not allow me to adjust shutter speed more than 1/250 sec. and I'm so confused!
Is there anything wrong with my camera or evaluation system? Or it's just me doing things wrong?
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FYI I know I can use other modes and you know, any other things you would advice, but please just answer my questions!
Thanks

[Waddizzle]

" Of course that leaves open the question of why it sometimes happens and sometimes doesn't." 

Of course.  What randomly occuring event is making the difference?  My guess is lighting.  I would be very interested in knowing the exposure settings in the two sample pictures, as well as the camera and flash modes that were used.

[diverhank]

I'm a little puzzled here.  If you set everything to manual with fixed flash power, fixed Av, Tv and ISO and I assume your strobes can't do ETTL...wouldn't it get darker when you move further away from the flash source?  Light loss is distance squared, right  ?(double the distance, 4 times the flash power).  If your flash can do ETTL, I guess it did not have enough power for the distance you had.