APRS ~ the packet is not recognized in some location

APRS stations near Brentwood

(There is one DIGIPEATER on Mt. Diablo)

  After testing APRS a couple different location, it seems that it is hard to send out the packet in some locations.   In my commute route, somewhere between Fremont and San Jose is one of locations.   I also went to Brentwood where might be a similar.
   For the commute route, it does not depends on the output power.  Even if I set the output power higher which is 8W instead of 1W.   I do not see much improvement.   It seems that it is just because more traffic.   From home to Fremont, almost all locations I sent was recognized, but after Fremont, probably a half of sending is missed.   A similar thing happened in Brentwood.   I tried to use 8W in Brentwood, it does not seem to help.   Instead of using higher power, it helped sending more frequently.   Then one out of ten was recognized.  In Brentwood, I did see more packets were going on.   If there is a lot of traffic, I guess using hand held might be some disadvantage.   Many mobile radio or base station might be more power something around 25W to 50W with probably a better antenna.
   It might happen when I use it on a trail.  Even if the location is better, it might be a similar situation.   So I really need to go hiking to try to see if it is still useful or not.   Originally, I thought it can be picked up if a DIGIPETER or IGATE station are reasonably close.  But it does not seems to be.