Maybe the quality of the plate is not good. Generally, if it's high-quality, it can be stored for 2-3 years.
The film is made from gum. Gum expand when heated and contract when cooled, which make it difficult to remove the film.
The film is sticked with stickers. The stickers may get wet after too long, which makes it difficult to remove the film. The appearance of the plate may be ruined even if you successfully tear off the film because it will look dirty with glue left on the plate. The glue is quite similar to cement.
There may be many reasons. One of them may be that the aluminium-plastic plate was stored for too long. And you dind't pay attention to keep it. Accoding to you, it's been stored for two summers. The glue coating of the film will be semi-melting and the adhesion of the surface of the aluminium-plastic plate will be stronger which may make it difficult the remove the film if the film is stored for too long.
If the film is not very large, you can slowly tear it off by hands with patience. Remember to do it slow. Try to make the angel between the direction you are pulling the film and the plate an angel of about 90°. You can also scratch the film using sharp knife and try to make the scratches parallel to each other and make the spacing between two scratches no more than 10 cm. Be careful not to damage the plate. You can try on useless plate at first. If you can't control your strength, you'd better not use this method.
In addition, you can try to heat the film with electric iron (Making sure that the temperature is not too high. It's all up to you.) and then slowly tear it off. And it will be better if you put a piece of cloth or paper on the plate when you do that. Of course, using electric hair drier is more convenient.
And there may be glue remains on the plate after you tear off the film, you can wipe it off with kerosene or alcohol.
