Have you ever heard of anyone saying that they sent their resume to a company but it seems to have disappeared into a black hole? How does that happen?
Let's ask some questions of the process.
Did you send an unsolicited resume to randomunknownhrperson (at) company.com? If it was not marked "spam", it may still not have made it to the real, live person who takes care of HR. Does the company have a person whose sole job is HR or recruiting? If not, the office manager probably reviews the emails to HR @ company. com and deletes them if the company is not currently hiring – your resume attachment may not ever get opened.
Did you send your resume to a resume blasting company? Those really, really rarely get opened. Did you send it to a recruiter or "headhunter"? Most recruiters will not open unsolicited resumes because the chances that you are looking for the position they are currently working on is microscopic.
In these cases you will probably not even get a "yes, we got your resume" auto-responder email.
If you send your unsolicited resume to someone who is not a personal friend inside a company, you may not get a response either. If it is to someone who knows you, they may acknowledge getting it and promise to send it along, but if they send it to HR, it will probably be deleted or trashed, unless the person you are sending it to is the hiring manager or a very senior person, and even then, it depends. If they pass it on to an actual, relevant hiring manager, but one who is not currently at the resume receiving part of the hiring process (either he has not yet gotten the budget to hire or is already scheduling interviews), it is unlikely that he will respond to a generic resume. Is the company actually in the process of hiring someone with your skills? How far along the process are they? If they have narrowed down the slate to the current top 3 they will not want to slow it down by adding you (unless your resume is amazingly steller and directly speaks to their current problem).
So an unsolicited resume, even given to someone who knows and likes you, is unlikely to get you an interview, and even less likely to get you the job.
If the company actually posts the job online or in a newspaper or magazine, they are soliciting resumes and applications. But check the dates. Sooner is always better than later, but you do not want to be the very first or more than a month after the posting date. The Applicant Tracking System is the name of the Black Hole in this case. Computers are dumb. They do not know a synonym from a homonym, never mind the name of the software that replaced the one in the job posting. If you misspell a keyword, they will not be able to pick it up. If there are 7 ways to say something and you do not use the one in the job posting, your resume will never see the light of day again.
What ATS (Applicant Tracking Systems) are good at is sending an auto-responder when you complete the application process. Filling in all the forms will be necessary, or into the black hole you go. It can take up to 4 hours with some systems and it will ask for all the same information on your resume plus other things you never thought of and will have to stop to look up. Another thing that the ATS system is good at is flagging how many times you have applied to work at the company. If you apply for jobs that really do not fit you, you will never get a job there. You will be flagged as a nuisance.
Only 20% of jobs are filled by job postings and some of these are guided through by contingency search people who go around the system and send your resume in directly to HR (see above). Most HR teams have notes on the career page of the company website saying that they do not pay contingency search fees. If a contingency search firm sends in your resume, but you had already applied and are in the ATS, the company will not pay the recruiter, probably will not look at your resume, and you may be flagged a nuisance.
75% of all jobs are filled by personal networking. This is when you are known, remembered, liked and trusted by the hiring manager and / or a member of his team. At that point one of them will ask you for a resume. It will not be seen by the ATS or perhaps not even by HR until after the hiring manager decides to hire you. It can have more than one way to say something and you will know what keywords, language and lingo that particular hiring manager uses to talk about what they are doing in that department. You will not want to apply to other departments because you will know where you fit. You will know that you bring the right skills to the table and know how to solve the problem.