[Astul The Shepherd]

Yep looks like lack of fertiliser to me.

It should say on back-of-pack what the re-application time frame is. It's usually 2 weeks for quick release and 6-8 weeks for slow release. I'd assume this is slow release as it's starter fert and not just N.

You'd be fine with slow release and the high P will be good for the new grass. The 30-0-0.3 doesn't have enough P or K for you. Follow the coverage instructions or use one of the fert calculators (or ask here) to figure out how much you should use.

I can't see from this distance but check your mower blades are sharp, it can make a big difference.