Guiding Bolt should use ranged spell attack so, assuming she is at ranged, she should have the same to hit chance as a fighter with 18 ST. In this case +4 (for Wisdom) and +2 for proficiency i.e. 1D20 + 6. To test this mouse over a target with a fighter (18 St) in melee and note hit chance and then do the same with shadow and it should be the same if 5E rules are being used.
BTW if you are not at range then use a touch spell.
Spells like Sacred Flame are different and require a saving throw by the target (Dex in this case), at wisdom 18 and level 4 the saving throw here would be a (8+2+4) 14; a goblin should have, as a ¼ challenge rating creature, +2 bonus (due to its Dex) so has a 40% chance of making the roll, hence she have a 60% to hit – I think Larian have played about a bit here and her chance seems less.
To be fair I have not done any testing, I am just going off my memory; please do the testing for me, I'm too lazy
To add onto this, if you are attempting to use a Ranged weapon/spell attack while within 5 feet of an enemy, you'll have Disadvantage on the attack. Which means that you'll roll 2d20s and choose the lower result, roughly equivalent to a -4 to hit (or -22%). Mousing over a target after you've selected the offensive ability should
reveal whether you have Advantage or Disadvantage on the roll.
About Sacred Flame: Larian's goblins have somewhat lower AC, higher HP, and the same Dex STs compared to 5e. This means that spells that target saves are relatively weaker than attack-roll spells that target AC. Additionally, spells that target enemy saves don't benefit from bonuses like Advantage or the +2 High Ground Bonus. So if you've been using Sacred Flame a lot as SH, then she'll feel weaker than other classes due to the relatively lower effective to-hit.
Otherwise, since you specify that she has 18 Wisdom which is expected for a level 4 character, I'd attribute your experiences to bad rng.