We'd not know why you might be having these difficulties. We'd have to see the receiver to find out. Please forward it to us so that we have the opportunity to determine what exactly is causing the loss of range.

Depending upon the type of foam, the vibration level in the model, and even the tie-down method, vibration can get into the receiver and shorten its operating life, but that's just speculation. 1/2" of foam should be plenty for almost all model aircraft, as long as it's not compressed in any way. If the tie-down strap is compressing the foam, then vibration may be able to enter there. If the vibration is abnormally severe, we'd also expect servos to be damaged. Since that's not happening, that level's apparently not too severe for properly-protected equipment.