Therein may lie your problem, setting the timing by the pins is only the factory standard set, which was just to get it sort of somewhere that should work but by no means optimised at all and from timing all the diesels I have had I can tell you the factory set is rarely that good!
Now you don't do the FIP timing adjustment on these by rotating the pump on the casing. It is a whole lot easier than that, you remove the rubber bung on the front of the timing chest and you will see the timing pump sprocket, you pin it, then loosen the 3 bolts a little then you can rotate the timing sprocket hub independently to the driven part of the sprocket. The pump will move easily, so you can then use smaller or bigger pins (drill bits) as a gauge.
I can't remember where I ended up leaving my timing in relation to where where it was, but for topic of discussion I started with say a 7.5mm drill and took it down to about a 6.5mm or a 6.75mm drill bit, I did this simply to give me reference to my adjustments.
The good thing here is this, if you make a mess of it, you put the locking pin (drill: 11/32) back in and you are back to where you started.