Checking brooms for curses

[hpistheman4me]

No: HPFGUIDX 40692

Yes, I can see how all that makes sense. If like Porphyrra 
says, "perhaps experienced wizards could tell by looking at 
the 'broom bucking' jinx that it wasn't something physically embedded 
in the broom", how come Professor Snape is the only wizard who tries 
to do a counter-curse? I have also been wondering why if Professor 
Snape seemed to know who was performing the curse why he didn't go to 
Professor Dumbledore? Anyway, I am only 16 years old and I don't want 
to ask questions that will seem stupid in the long run. Well thanks 
for answering my question!
~Cindy~