Thanks, going to bulk prime next time. It is cold here so maybe that is the problem.
Yep, we're experiencing that cool thing even up here in parts of sunny Queensland, perhaps you're not heating them for long enough? I let them go for up to a week somewhere warm and then move them into the pantry or chiller. If you have an electric hot water system indoors, pop your bottles on top in a box or crate for at least a few days. The extra heat should kick priming along, just don't be letting them get too hot, I aim to keep my ales around 20degC (an infrared thermometer makes measuring their temp a piece of piss BTW). Add layers of cardboard help to regulate the heating or a blanket over the top to hold more in. Failing that, in a box in the sun during the daytime perhaps (keeping light out but heat in)? In the pantry or an esky even with a hot water bottle? Electric blanket? Back in your ale fermenting fridge etc... you get the idea.
I'm usually a bulk primer but I've had a few uniformity issues too, am finding that it needs i) racking on to the dissolved sugars, ii) gentle whirlpooling/ agitation from the racking hose but not aeration (which will cause oxidation), and iii) time to diffuse completely, say half an hour or more.
The uniformity problem that I've experienced has been, over the last say ten or dozen batches since I've begun bulk priming, one cracked bottle (i.e. nearly a bottle bomb, the glass just let go in place but it didn't explode), a handful of gushers, most of the rest were fine, but a few nearly flat too. If it was just that most of them were fine and some flat (like your situation), I'd be suss on either the bottle caps or temperature, but seeing as with mine a few went the other way, I'm tending more towards non- homogeneous priming sugar distribution, so when you start bulk priming be very careful of that. Just don't over do the agitation bit else it'll end up oxidised and taste like wet cardboard.
Having said that, I've found the Geocities calculator undercarbonates if using its style guide. I'm not slagging it at all- I use it all the time- but initially I was disappointed by the level of carbonation I was getting. I have increased the desired volumes by up to 100% in some cases, although it could be that my expectations for the styles that I've done batches of are different. As an extreme example, I have used 130g sugars in 23 litres of English Bitter at 20degC, which equates to 2.4 volumes as opposed to the guide of 0.8 - 1.3 which takes a maximum of 40.5g. You need to be very, very careful in doing this, I have only done this after a fair bit of experimentation, and recommend only using PET bottles for trialling different rates.
[Damn, that was almost another essay- sorry...]
