liftmaster 3500D problem

Two car garage opener (Liftmaster 3500D) has been in (light) use for four years. Vehicles are sedan and mid-size pickup. Has always worked OK with sedan on both sides of garage; has always hesitated ever so slightly with truck on both sides of garage but more so on the right (facing door from outside)side. Please note that ALL other functions work perfectly and quickly with BOTH vehicles in ALL locations.Lately the operation when signaled from the truck has taken 'spells' (sedan continues to work OK); opens door for a week or so and then door will not open and has to be 'coaxed' by repeated 'requests'; eventually the door opens (so far). Please note--and I apologize for repeating this--that ALL other functions work OK from the truck in ALL locations with NO 'spells' of not working.
It boils down to this: Prompt entrance to garage by truck is unreliable.
We, and two "experts", have tried everything to no avail. By the way, LM 'phone/on line help has been useless; they don't even seem to "get" the problem.
Any suggestions?

Are you using the remote that came with the unit to run the opener or are you using a Homelink system (where the buttons are built into the vehicle and the code is cloned from an existing remote)?

Both vehicles are set up with Homelink. Sedan works fine; when we approach the door with the truck the response is unreliable (sometimes immediate, sometimes greatly delayed) but eventually the door opens if we try multiple times. (However, we don't always have the patience to wait tat long).

We are currently trying out an 'expert' suggestion from another source: An extension of the antenna to the outside. So far, the door has opened promptly. We will have to test this for at least two weeks in order to document reliability since prompt truck operation of the door has been intermittent (we may just be in am OK operation interval before things get stubborn again).

Other measures tried: Removal of baseboard extension that was rubbing severely against bottom edge of door; lubricating the vinyl seal (it is rather stiff in cold weather); varying position of truck when sending signal. This last is really impossible to document; there is no consistently 'better' position.

By the way, we like this opener; it's very quiet compared with most and has presented no other problem.

Thanks!