You need to run the randomize function to seed the random number generator.  
I would use time as the seed. the help below says that the randomize function
is started with time.  Maybe they is something wrong with your system timer.

Randomize (Time)
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VBA help for randomize

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Randomize Statement
     

Initializes the random-number generator.

Syntax

Randomize [number]

The optional number argument is a Variant or any valid numeric expression.

Remarks

Randomize uses number to initialize the Rnd function's random-number
generator, giving it a new seed value. If you omit number, the value returned
by the system timer is used as the new seed value.

If Randomize is not used, the Rnd function (with no arguments) uses the same
number as a seed the first time it is called, and thereafter uses the last
generated number as a seed value.

Note   To repeat sequences of random numbers, call Rnd with a negative
argument immediately before using Randomize with a numeric argument. Using
Randomize with the same value for number does not repeat the previous
sequence.