Hi amy arabica
I dont have the same machine, but Im a pressurized basket/supermarket pregrind user too. *Ive noticed a big change in crema depending on the pregrind - freshly open packets give a decent result, totally stale gives next to nothing. *Try changing your brand of pregrind. *Make sure that the brand/grind youre using is designed for home espresso machines, not just labelled espresso - some arent and are better suited to plunger. *I played around with a few brands and settled on vittoria oro as one that will give a decent, consistent result (although as always with the supermarket stuff, some batches can be dodgy). *
Tamp could also be the issue - it has to be firm enough to not allow any channelling.
Could also be a heat issue - make sure you warm up the PF/basket with the machine, AND your cups. *All makes a difference in my experience to the quality of the shot you get from pregrind.
hope that helps *:)