In Linux /dev/random is a special file which serves high quality pseudo random numbers.
This implementation collects entropy from events originating from the keyboard, mouse, disk and system interrupts.(refer this document) So when there are no such events, the entropy pool is empty, reads from /dev/random will block until additional environmental noise is gathered. This explains your problem. To fill the entropy pool you can press keys on keyboard.
On the other note a truly random number generator uses Hardware random number generator which generates random numbers from physical processes.These processes include microscopic phenomena that generate a low-level, statistically random "noise" signal, such as thermal noise or the photoelectric effect or other physical phenomena. These processes are, in theory, completely unpredictable, and the theory's assertions of unpredictability are subject to experimental test.
A hardware random number generator typically consists of a transducer to convert some aspect of the physical phenomena to an electrical signal, an amplifier and other electronic circuitry to increase the amplitude of the random fluctuations to a macroscopic level, and some type of analog to digital converter to convert the output into a digital number, often a simple binary digit 0 or 1. By repeatedly sampling the randomly varying signal, a series of random numbers is obtained.
The Hardware Random Number Generator gathers environmental noise from device drivers
and other sources into an entropy pool. From this entropy pool random numbers are created. When read, the /dev/random device will only return random bytes within the estimated number of bits of noise in the entropy pool. This explains your problem.
Some implementations of Hardware RNG are explained in kernel doc and information on a device.
A counterpart to /dev/random is /dev/urandom ("unlocked"/non-blocking random source) which reuses the internal pool to produce more pseudo-random bits. This means that the call will not block, but the output may contain less entropy than the corresponding read from /dev/random.
So if your intent is not to generate CSPRNG(Cryptographically secure pseudorandom number generator), you should use /dev/urandom.