Have used these joiners for years. Always work well. If they don’t fit - Check your track ends - It won’t be the joiner!
To aid fitting make a simple tool from a scrap piece of code 100 rail. Bend the top surface of the rail to an angel of approx 30degrees.
The angled piece should be about 50mm and the remaining piece about 20mm. Tape the angled section to form a grip. Slide half a joiner onto the 20mm long ‘tool’ and engage the remainder onto the end of the track to be used. Hold in place with angle nose pliers and remove the tool. BEFORE fitting any joiner to cut rail it is ESSENTIAL to de-burr the underside of the rail being fitted and the top side of the
‘flat bottom’ rail section too.