The biggest issue I see again and again is time. Hiring good people is hard. Hiring good people fast is really hard. Hiring lots of good people fast is really fucking hard.
Ultimately it takes a huge amount of time to source, filter, screen, interview and hire good people. A lot of early stage founders complain that hiring ends up being their biggest time sink. As a result, right now in London (UK) there's an enormous demand for internal talent people to come on board and take over that responsibility as external agencies are an inefficient and expensive solution.
Putting time to one side, my biggest issue is not so much the finding, screening or interviewing part, it's the engagement part. Give me a job brief, a short chat with the engineering lead and a couple of hours in front of my laptop and I'll create a significant list of potentially appropriate candidates. even if the engineering lead reduces that list by half, convincing those people to have a conversation is an incredibly difficult thing as the majority of 'good' engineers are already in work and know that there is a world of opportunities out there if they could be bothered to look so why should they care about you and your random opportunity?