Understanding the role of 2 capacitors in a pi filter
You are feeding the filter from a zero ohm source. 

Ask yourself what the ratio of C1's impedance to the source impedance is, and you'll see why it appears to have no effect.

Introduce a 150 ohm series resistor (to match R1) between V1 and C1 and it'll start to make more sense. 

Any time you see a Pi filter, work out what the source and load impedances - terminations - are. The filter is designed with these terminations in mind, and if mis-terminated, it won't have the expected frequency response.
Usually source and load terminations are the same, but sometimes the filter is designed to work from a low source impedance into a high load impedance, to avoid the 6dB loss (half the voltage) of a normally terminated filter.