True 0 movement [Eureka Mignon Especialita]

Hello all,

I bought an Eureka Mignon Especialita some months ago and it has been working really good.

But last 2-3 weeks it has been difficult to dial in my espresso.

I set the zero when it came but now I checked again and it has move to -1'5, is this normal ? I need to reset the 0 form time to time ?

Thanks to all!