FL STUDIO12 HELP? step sequencer issue?

A step sequencer works in a fixed time schedule; that's the fundamental principle.

What you want to store a sequence with "live" timing is a looper.

"looprecorder" works well for that. I use it in Tracktion.

https://www.4drx.com/plugins/looprecorder/

(T6 is available totally free and unrestricted; it's the full program from two releases ago https://www.tracktion.com/products/t6-daw )

If you want to stick with a sequencer, Hyseq16x3 is pretty good. You can't use actual recording, but you can add some randomisation to the timing of each note and set a percentage probability that each note will be played or not, so it sounds much less mechanical than a "pure clockwork" sequence.

The free version is half way down this page:

https://hy-plugins.com/product/hy-seq16x3/

That outputs MIDI so it needs to feed either a synth or sampler.

TX16WX is an excellent sampler and totally free (see the links below).

The sequencer is single-instrument-at-a-time - but you can add that an a synth/sampler VST into each of several tracks to give multiple instruments (or you can in tracktion, anyway)..