You do not have to worry so far. Lawn shoots best at a constant temperature of about 14°C. With increasing temperatures hopefully the remaining lawn seeds will shoot as well. By the way, it is also a sign of quality lawn seeds when not all seeds shoot at the same time. You then have a good mixture of seeds which will guarantee a dense and strong lawn later on. Please abstain from the fertilising right away the next time you sow lawn. That will help to build the roots properly. If you want to fertilize, wait at least till after the first mowing.