vinyl plank flooring is separating

[Sol Bergren]

I've installed vinyl plank flooring in a rental, I really like it but it keeps separating (I helped install this with someone who has installed alot of flooring previously but never the vinyl plank type, so maybe we did something incorrectly).  

Has anyone else had this problem, I've removed the baseboards and tapped it back together once already but it continues to separate, not really sure what to do, it looks great and holds up super well, except for the gaps!  Anyone have suggestions or encountered this problem??