Trying to get a patent on your invention should not be as hard as it is. You might be surprised the first time you try to file a patent and you have umpteen pages of documents that need to be filed, fees to be paid, and deadlines to meet. Even if you do all of the above right, you might still encounter some common issues with patent filing. You may need a patent lawyer's help to resolve the following issues.
Suing for Rights to Your Invention
If you did your due diligence and researched patents and patents pending for any products and inventions that might be remotely like yours, then you should maintain full rights to what you have created. There are other inventors who may try to claim rights to your invention, or the patent office may claim that someone else filed a duplicate patent ahead of you. In both cases, you would need the patent lawyer to investigate and file a lawsuit for rights to your invention and the right to file a patent on your invention. Patent litigation in this area is quite common, since ideas spoken aloud or demonstrated to others can cause theft of ideas. (It is a valuable lesson to learn, at any rate, to keep all future inventions "mum" until you have a patent on them.)
Being Sued for Your Invention When It Is Similar to Something Offered by a Company That Allowed Their Patent to Lapse
Companies who had a patent on a similar item to yours and allowed that patent to lapse may try to sue you for recreating and/or enhancing said product. The lawyer you hire would then be defending your position as one that shows that your invention is an improvement on the one that company sold and that you have a right to manufacture and sell your improved product because the company suing you allowed their patent to lapse. In this case, you absolutely should hire a lawyer, seeing as you are being sued by a company over rights to two similar products and the patents pending and lapsed. It would be far too complicated a case for you to avoid hiring a lawyer, and you may lose if you do not have a lawyer defending your position.
Existing Patents with Your Name Attached
This is a case of patent fraud. Someone not only stole your idea/product/invention, but he/she also stole your identity. You will need to alert the patent office to this issue and then pursue the case with legal help to make sure it is remedied in time to avoid a complete stolen identity.