Solved

DRS

The last couple of races I have qualities really well, with one driver on pole and the other in the top six. 

In both races I have been mullered in the DRS zone. The rest of the track I’ve done well, but in the zone cars have been coming from a long way back and passing me. When reversed I’m struggling to close the gap. 

Is this just a better developed HQ? Or is there another obvious reason?

If you're only talking about DRS difference, yes it's just a matter of higher Technology level.

Your level 10 against 15+ level players and that makes a huge difference in DRS improvement. Over a race distance this can all add up to quite a few seconds especially when you then add on the extra KERS time that each level gets. Unfortunately its something you will need to put up with until you can level up. Consider your doing a good job if your competitive now even being many levels behind, it will increase your reputation.

Thanks. So what is the best developments to concentrate on with my HQ?

Design and Technology are the most important ones in terms of speed and overall performance.

Yeah I'm in the same boat, in rookie it doesn't seem to matter much but once pro hit you have a few teams who are higher levels. It's pretty hard since you take 1-2 laps to close to gap then make the pass but unless you can put a 1 sec lead on them in a single lap they'll zoom right back past you the next lap. 
I tried a couple things lately with varying degree of success. Making a quick first Stint on SS to avoid getting destroyed once DRS and KERS happen. That way I'm pitting earlier and I'm "offbeat" with them... worked sometimes. of course trying to pass them onthe very last lap usually work but that is only 1 position. Rainy races are a blessing though! I've won some of those. And Monaco is great too

I know from the research screen that my car is pretty close to the leaders in all 4 "big 4" stats. 
I focused on the design facility as I thought having more points would mean a faster car, if I am faster no one will be within 1 sec so who needs technology... but research seems very strong. 

So I too switched to technology for now.... it seems it's just a matter of leveling up and developing these to catch up for us. Feels sad when you are .500 behind and after the KERS you are still .250 behind while they go from .999 behind to .500 in front.