The number one rule of troubleshooting is: Do not randomly replace parts.
Just don’t do it. Not even one. Never replace a part in a design that you know has worked in the past until you have verified either in the circuit or out of the circuit, that part is actually defective in some way. New parts, unless acquired from some Shenzhen bargain bin, are not suspect. It is your work that is suspect.
Unless, the original component manufacturer has changed the specifications since the circuit was designed!
"Been there, done, that, got the tee-shirt!"
On one occasion, I was doing some mods on a TV Transmitter remote/auto control system.
This unit used several monostable multivibrators to provide a set of delays.
It was convenient, whilst modifying an unrelated part of the unit, to remove these devices.
Unfortunately, one fell on the floor, & as is not unusual in such cases, "disappeared from the ken of man!"
"Who cares?-----plenty in the store!"
I grabbed a couple, stuffed them in the sockets, & proceeded to complete the mods, & test the control unit.
It didn't work!
It turns out that the original chips of that part number had been found to be unreliable at providing short delays, & had been redesigned to optimise such use.
We, however, were using it to provide a long delay, in which use the modified device had become prone to early "time out" when using the same CR time constants as before.
The IC maker, (National Semiconductor, if I remember correctly), foresaw this problem, & produced a new monostable, similar in all respects, except optimised for long delays.
This, of course, had a different part number!
No Internet back then, so after a lot of research, I found about the change, purchased the new chips, tested the control unit, which now worked properly, then changed all the schematics & documentation to reflect that change.
This was an"in-house" design, with only two such units in service, so we could "cover all bases".
Just imagine if there had been thousands out there, with an ongoing production run!
So it’s a kit. Even more reason to blame your work. The sooner you accept the fact that with a kit, 99% of the time, it’s something you did wrong, will you become more successful at kit building.
Did you install the parts in their correct locations? It is incredibly easy to swap a 2.7k resistor with a 27k. Did you install an electrolytic cap backwards? A transistor, a diode backwards? It is very hard to look at your own work objectively. But, that’s what you need to learn. It can be difficult. Everybody makes mistakes. Everybody.
Then of course there is your soldering. It’s probably way worse than you think, especially if you’re using a cheap iron and lead free solder.
The most important things you can post are:
Photographs. Of your board. Top and bottom. Well lit and focused.