This can only happen for a few reasons. Keep in mind that proper maintenance of the vacuum you are using will have a great effect on suction, and that the sander creates no suction, all of that comes from the vacuum as the sander is simply a pathway.

First, check to see that the holes in your abrasive are in line with the holes in the red sanding pad. Next, make sure there are no holes, cracks, kinks, or damage to your vacuum hose and that both ends are securely attached (the one going into your vacuum port and the other going into your sander).  

Check to make sure that your hose is not clogged with debris, that the collection bag in your vacuum is not full and that your filter(s) are clean and not blocked or damaged.  

One other reason that this could occur is that when sanding, your sander is not maintaining full contact with the surface you are sanding or when the sander goes beyond the edge of your substrate. When this happens, the suction “breaks” and becomes less because one or more of the pre-cut holes in your abrasive has extended beyond the edge of your sanding surface.  

This can also happen on curved surfaces. To remedy this on curved surfaces, use one of the foam interface pads with your film, sandpaper, or screen or use the foam abrasives. Both will allow your abrasive to better conform to the surface you are sanding and maintain maximum suction for dust-free sanding. 

Your sanding cadence or speed of sanding, if with rapid back & forth movements, will not allow enough dwell time for sanded material to be removed efficiently and can build up momentarily until the sander passes over the previously sanded area at a proper speed. Â