Laser Machine Cutting Problem

I have a Chinese 50W laser machine which i bought very recently. I just noticed this cutting problem with the X axis. I just gave a speed of 5 mm/s to cut a letter on a 3mm acrylic plastic sheet. It worked slow and perfect until it reached the horizontal axis which increased the speed and hence all the horizontal lines were not cut. I have attached a picture of the backside of the sample I just tried. Please help. Thank you in advance.
Attachment 271278

Are your mirrors all aligned

This is lens aligment problem. Also check your focal lenght.

How does the mirror alignment increase the speed of the laser head while it moves horizontally?