i'll tell you two tricks i have learnt.
If you save half of the lemon that you have scraped the pulp out of and then stick the complete half in on top of whatever you are cooking and let it bubble away for the first 15 or 20min before you use tongs to take it out then you get more pectin in the mix without actually adding more lemon. i did this to a couple of jams i made recently but can't for the life of me remember which ! i need to get into the habit of recording as i cook cos i never seem to exactly follow a recipe. It may even have been this jam....
<insert embarrassed emoticon here!>
OR
there are heaps of other jams that you can 'hide' an apple in. It won't effect the flavour and it adds a lot of pectin. I have actually had a heap of success with any jam i have made that i added some apple to. I buy a bag of the big granny smith juicing apples that are really cheap. About 3/4 of an apple weighs 200g and that is how much i put in a regular full batch of jam. Don't peel it and leave the seeds in. I put the apple in first (or after the zest if i am adding citrus zest) and wizz it up for 5sec on speed 7, scrape the sides and do another 5 sec.
Recently i made chiili jam and also a sweet beetroot jam and neither of them have significant pectin (from what i know! i could be wrong) but the addition of the apple made them perfect.
But i do have a packet of jamsett in the pantry and if something looks like it is not going to gell after 45min rather than cook the crap out of it i add about 5 or 10g of jamsett and cook for another 4 or 5min.