There can be two reasons for a lapel that won't lay flat.

First, the lapel is not cut correctly. This may be your problem seeing as your jacket was altered. The lapel would need to be re-positioned. The tailor normally opens the lapel and recuts the interfacing inside to allow the lapel to lay in the new position.

Second, your lapel may need pressing. If your tailor correctly re-located the lapel perhaps it has not been pressed correctly.

To do so you will need a Ham.
These a haggis shaped cloth, stuffed till firm, and used to press complex shapes.



Place the Ham under the lapel and cover with a slightly damp cloth. The moisture in the cloth will form steam which needs to be forced into the fabric to soften the fibres so that they can take the complex shape.

Do Not skip the pressing cloth!!! It prevents the wool from getting shiney.



Using your steam Iron a lot of heat and pressure you are going to force steam into the fabric, press with HEAVY pressure the fabric into a gentle curve, and heat the fabric till it shapes and takes the curve.

Then allow the fabric to cool without moving it. Voila! a perfectly curved llapel.