Improving RF range

[s8utt]

I've been increasing the number of sensors I use over the weekend. I have 2 USB rfxmodules

1 433mhz
1 310mhz

On the 433 mhz module I have the following:-
2 x RFXcounter modules.
An Oregon rain meter
4 Oregon temperature sensors

On the 310 mhz module I have about 10 MS13 detectors.

I've noticed that the rfxcounter and one of the temperature sensors I pick up very infrequently. Moving the temperature sensor closer to the reciever and the readings are caught a lot more frequently ( I'm using the rfxreceiver program )
The rfxreciver program doesn't show any unrecognised readings so I think its just not picking them up.

I have 2 thoughts so I was wondering if someone can see if any have merit.

1. The computer is quite old that I'm running the rfxreciever program on, there can be sometimes 2-3 seconds between the recordings of the first and last readings from the rfxreciever ( 5 readings ) Could the computer be too old to cope ?
2. How can I improve the ariel, my recievers are quite old, they are before the recievers were fitted with telescopic ariels. at the minute the ariel is on in the attic ( centrally ) and I'm having problems receiveing the signals from the extremes of the house on both ground and upstairs rooms. Would I benefit from a new ariel, a better placement, two ariels, fitting an amplifier to the ariel ?
3. Could it just be signals are happening at the same time so only 1 is being received ? Looknig through posts this seems doubtful or just unlucky, I only have 6 (433) devices transmitting. Some poeple have quite a few more.

I'm leaning towards option 2, so I was wondering what has the highest success rate ? Anyone been through this already and can offer an insight ?

Many Thanks
S8utt

[b_weijenberg]

Position the groundplane antenna horizontal so that the sensors below the antenna are received better.
Position the RFXMeter (I guess this is what you call a rfxcounter) also horizontal. Cut the antenna wire inside the RFXMeter at the cold side (close to module 3), drill a 2mm hole in the top cover close to the green LED and position the antenna wire straight from the RFXMeter.
Position a RFXSensor also horizontal.

[s8utt]

bert as always thanks your a star.

Only moving the antenna into the horizontal position made a fantastic improvement

ps. yes i was talking about the RFXMeter