There are all sorts of reasons why your application to register a mark might fail.
The official who examines your application ("the examiner") may consider that your mark falls within a number of statutory exclusions. For a start, the examiner may consider that the subject matter of your application is not even a trade mark, possibly because it is not a sign, or perhaps because it can't be expressed in writing or doesn't distinguish your goods and services from those of your competitors.
Even if it is a trade mark, the examiner may object to it on the grounds that it is not distinctive, that it is descriptive of the goods or services, or common to the trade. He or she may consider your proposed mark to be offensive or it may be too similar to a royal, national or other protected emblem. Those objections are called "absolute grounds for refusal" and are set out in sections 3 and 4 of the Trade Marks Act 1994.
Wherever possible, the examiner will give you an opportunity to overcome the objections. If you fail to take advantage of that opportunity or if the examiner believes that the objection cannot be overcome he or she may refuse your application.
If you think the examiner is wrong you can appeal against his decision to the Registrar of Trade Marks (that is to say the Comptroller or Chief Executive of the IPO). The Registrar will appoint an official called "a hearing officer" to consider your appeal. I have described the appeal procedure in "If the examiner says "no" - ex parte hearings in the Trade Marks Registry" 10 Aug 2015 NIPC London. That is where I or some other specialist intellectual property barrister can often help. We can advise you of your chances of success and, if necessary, represent you at the hearing before the hearing officer.
Even if the examiner has no objection to your application or you manage to overcome any objection that he or she raises, you may not be out of the woods. Someone else may object to your application and I will consider that problem next time.
Until then, I leave you with my case note on T-397/09 Prinz von Hannover v OHIM  EUECJ T-397/09 (Trade Marks: Prince Ernst August of Hanover and Brunswick etc v OHIM 6 June 2011 NIPC Law). Cheeky wasn't he!
Enjoy the lovely weather folks.