Reception on Vacuum tube car radios

Here's a question I have about a something that I see quite often. I repair a lot of old 6 and 12 volt car radios (tube type). Sometimes I'll have a radio that only picks up when I'm holding the antenna in my hand and it uses me as an auxillary antenna. I've checked the RF amp tube and circuits, swapped the tubes etc. I've tried different locations within my house, or usually I test it out in the garage where most models have great reception. But some still only brings in stations when I'm holding or touching the antenna.
Any suggestions on why this happens? And... if the radio was mounted properly in the vehicle with an antenna would this still happen? What I mean is, would this be normal for some models?
Thanks in advance for your replies.

Well for one thing you do not have a balanced antenna system when the radio isn't installed. Your hand has some influence because of the body's saltwater content. You normally wont find an antenna with that much mass.

Also sometimes when touching a grid-cap you will notice a non-operating radio will start playing. This is due to changing capacitance and stray static voltages produced by the body.

The car body has a certain amount of metal that can act as a counterpoise antenna (all antenna systems require a grounding system) Most radios have a padder cap that can be adjusted for the length of the antenna, I remember in the Chevy radios the padder was adjusted by removing the volume knobs from the front of the radio. Most others usually have it on the back inside a hole in the metal next to the antenna jack. Try tweaking on those to see if you get better reception, that should help.
PS on the bench, hook up a long length of wire to the chassis to act as a counterpoise.

Thanks. I thought I had done that but I must not have. I have so many in the hopper I lost track of which ones I had done what to. I tweaked the antenna padder and two others inside and it blasts like mad now! Thanks again.