with suckers that are making the lilac very bushy, i d like to just which branches that should cutting to make the flowers more prettier
Be sure to prune AFTER the lilac blooms and not before (unless you are rejuvenating). Here is more:
The Bush next to it is beautiful.
This is a difficult one. There are a few things that I could think of, though.
There is a chance that the one next to it has a bigger root system, and it is out competing this one.
There could be a chance that the shrub is in slightly different soil conditions. Maybe it could be at a lower level and getting too much moisture.
One last thing could be an infection that just so happens to spare one and attack another due to slight genetic variation.
Your local extension service can help further. You can even take samples to them. This will help them to identify the issue. This article will give you information about extension services. At the bottom will be a link to find the closest one to you:
https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm
This article will give you more information on the care of your shrubs:
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/dwarf-lilac-varieties.htm