Interestingly, it's not the finer components that are the problem - more the larger ones.
Your caster is casting into a flask held at a high temperature so the metal reaches and fills all the fine detail before solidifying. The problem is that this is too high a temperature to get good results for the heavier items, they'll have problems with porosity.
Components of less weight and cross section need to be on different sprues so they can be cast in different flasks cast at different temperatures, as described in my earlier post.
There's nothing the caster can do other than what he has done.
Have a look at the pictures of the wax trees shown in the links on both mine and Andy's posts and see how the items are arranged around the central main sprue. You'll see that the items are not large numbers of components off a largish sprue that is then attached to the main central sprue but individual items attached directly to the main central sprue.
That's no good for what you want, the components in the pictures are larger than most of yours, but what you can do is arrange say, five or six small components in a sort of flat fan with each item on a short sprue linking at the base of the fan. The caster can then arrange these fans around the central sprue. It's a more efficient way of filling the flask and so should be cheaper.
Larger items like the springs benefit from extra sprues but arrange them carefully, for example with the springs have a sprue to the centre of the spring but then one either side going from this central sprue to the end of the springs. If there's more than one spring on the fan you can also take them back to the central sprue of the fan. Just have a think about what will allow the metal to get to the end of the component quickly.
The principle is simple, thick to thin, to thinner by the shortest route.
Not always possible with the larger components but that's OK as long as the rest of the feeds are arranged on that principle.
What can be impossible to cast well are fine details, like thin handles, on heavier items. It's not possible to get the flask temperature right for both the thin handle and the body of the component so they need to be made separately and joined later (if possible).
This sort of work can be a bit of a pain for casters. Normally if something goes wrong the caster just creates another wax from the rubber mould and does it again, at little expense to the caster and the customer is none the wiser. With 3D printed patterns the cost of the patterns can be relatively high for the customer and the caster only gets one go at it. If it goes wrong you get an unhappy customer so the caster might want to cast at a higher temperature than he/she would like to make sure the mould fills.
