Hertfordshire,
United Kingdom 
We have a well established Philadelphus / Mock Orange in our back garden. Earlier this year it was pruned quite drastically (having been neglected for years!) Recently we've noticed a few new branches dropping off. Also lots of what look like blackfly (maybe) on some of the branches. We didn't get any blossom this year (unlike in previous years when it has often blossomed profusely.)Overall however the shrub looks pretty healthy though. Should we be worried and should we be treating it? Will we get blossom again next year?
24 Jul, 2013
Answers
I suspect the heat and lack of rain. the new growth isn't 'woody' yet so will scorch easily and lose lots of water compare to the older stems.
Pruning as mg says is the trick to renovate an older shrub.
24 Jul, 2013
I had blackfly on my virginal this year, a quick spray got rid of them.... generally a lot more greenfly/blackfly about this year. ;)
26 Jul, 2013
Hi Helen and welcome to GoY, I suspect you pruned off the flower buds. They are group 2 for pruning and should be pruned just after flowering cut back flowered growth to strong young shoots lower down. Each year cut out up to 20 percent of ageing stems to near the base. Not sure why the new branches are dropping off perhaps someone else will be able to advise.
24 Jul, 2013