I come to you for some help

I am GMing a DH group and it seems to be going decently, except for the techpriest.

He has a good spread of skills, medicae, tech use and some common-lore but it's been happening that every time it's time for him to shine (tech use to identify the purpose of a servo-skull, medicae to determine the cause of death) he'll fail the roll and what was meant to be a moment for him to feel useful and shine becomes a moment where he look and feel like an idiot.

Am I being too hard on my tests (medicae test at +0 because the body is quite old) or is it just bad dice? Excluding combat encounters the poor guy failed all by one of the tests he made.

I brought this up before and someone mentioned there being rules for using time to help add modifiers to tests, but I can't find this in the core book

Well, thats the essence of a D100 System, you can be as good as you want, in the end you just highten your chances to suceed.

May I ask how much Intelligence he has and what level he is? May he take the +10/20 Tech-Use Skills? A Combi-Tool or an utility Mechandrite?

A test will never ever be as simple as +0 total modifier, skill levels, Talented<SkillName> talent, situational modifier, equipments bonuses or penalties… these all add up. Using your example, the body may be a bit old thus making it +0 rather than +10, but with a medical lab or equipment at his disposal, that could become a +20 bonus for a total of +20.

While it sounds like you applied modifiers to the test, a +0 test isn't something that you should expect a PC to be able to make by default.

The good thing about this system is that there are all sorts of modifiers you can find, or even make up. Even the base difficulty of the test can be set on the fly (and in such a way the players can't really object to).

I must admit I quite often put a test down as 'chellenging' +0 as default but I would start encouraging your players (including the techpriest) to begin looking for reasons to get positive modifiers. This is what I do. So I say "you need to make a tech-use test to restart the cogitator" and they then remind me of all the reasons they think they have an advantage.

Makes less work for me and encourages them to be creative, make preparations for things.

Concerning bonuses for actions your players need to use their head to help themselves.

You fix a difficulty for the action but it is your player job to find tools, lab, favour to expert NPC, research roll that can be used to help with the action roll (like reading user manual before trying to get it working, found details in library or data store,…).

Maybe you could give your player some clue about alternate ways to get knowledge prior to actions.

But really important point, common work is +10.