Putting a new missile or bomb on a warplane is not a simple process. It takes months, or years if there are problems. There’s lots of opportunities for problems. The process begins with a lot of calculations that determine, in theory, if the new weapon will fit on the aircraft and be able to operate safely and effectively. There is software for this.
Then comes the more hands on steps. First you need to ensure that you can hang the weapon off the wing of an aircraft. That may involve modifying some attachment hardware to make it possible for the weapon to work on the external pylons that extend from underneath the wing or the internal bomb bay. Once that is done you have to conduct wind tunnel tests to be sure that the missile will stay attached in flight if hanging from pylons. If that works out (even if modifications to hardware are required) the next step is to attach a missile (with explosive items like the rocket motor and warhead replaced with inert items that are the same shape and weight) and fly the aircraft at speeds and maneuvers typical of combat. Once that is certified to work in practice you have to deal with the electrical and software issues so that the pilot can activate and launch the missile. This process includes integration with the aircraft’s fire control system. That then has to be tested when installed in the aircraft, and finally there are a series of tests where first you test on the ground to ensure that all the electrical connections work, as does the software. Finally there are a series of tests in the air which first involve just releasing a live missile, then releasing and firing it.
Each of these tests may reveal problems that have to be fixed. These days there are usually few problems because missile and bomb manufacturers design their weapons to be used from a wide array of aircraft, even if the weapons do not go through all the tests and tweaks for it to be used on a specific aircraft. The configuration and simulation software has also gotten better over the years. Things can still get interesting if you have to adapt a weapon to operate on an aircraft that it was never expected to operate from. That has been encountered since the Cold War ended and some Western missiles and bombs have been adapted to operate from Russian warplanes. This is not impossible, just more time consuming and costly.