I guess summer is here... A/C questions are popping up...

I was waiting for hot days to try adding freon to my wife's car. (A/C system has developed a slow leak over the last few years and I had to add a can or two for hot summer months only to watch it gone the next spring. I've done this the past three years and was hoping I can get away with this once gain this year.) Well, last weekend I decided it was time. I had an extra can of freon and I have a manifold gauge which I use for refilling/adding freon though the low pressure side.

I started the car, turned A/C to max, closed windows, and connected manifold gauge to the low pressure side. The A/C compressor was cycling on/off every 4-5 seconds. The gauge showed the pressure between 20 and 45 psi during the cycling. But since the air lowing though the evnts was warm, I thought the I still needed to add freon. So I connected the can, pierced the opening in it, and opened the valve. I was waiting and waiting and waiting (maybe 15 minutes or so) but the readings on the gauge did not change: still between 20 and 45 psi and still cycling on/off every 4-5 seconds. Also, the can did not seem lighter -- I can feel gas inside the can. I figured the freon was not going in the system. As the last resort, I connect the can with freon directly to the low pressure side bypassing the manifold gauge. Still, after waiting for 10-15 minutes I did not notice that the can got any lighter. Why didn't A/C system accept any freon? I used exactly the same procedure the last three years and it was working just fine. What could be wrong this time? Any ideas?