English Civil War questions John Lambert's army

After Oliver Cromwell died people in England were looking for someone to take over. General George Monck had an army in Scotland that no one knew who he supported. Parliament sent General John Lambert to find out.
However, Wikipedia says “Lambert was now sent with a large force to meet George Monck, who was in command of the English forces in Scotland, and either negotiate with him or force him to terms. Monck, however, marched southward. Lambert’s army began to melt away, and he was kept in suspense by Monck till his whole army deserted and he returned to London almost alone”
Monck’s Wikipedia entry mentions Lambert’s army not being paid. Why did his army leave, why did they not get paid, and how did Monck’s army get paid if Lambert’s army could not.

Somewhat different to the detail in his Biography.

After Cromwell’s death factionalism was a serious problem with open fighting here and there. Lambert was regarded with deep suspicion by many as someone who was possibly looking to replace Cromwell as Lord Protector ( though his real motives are opaque ). In particular Thomas Fairfax, Cromwell’s former commander, was not a fan. It was Fairfax who played the crucial role in suborning some of Lambert’s troops - at his intervention most of the ‘Irish Brigade’, maybe 10% of Lambert’s force, went over to him which started the disintegration. By this point Lambert’s rear was compromised as noted above as his semi-ally Charles Fleetwood had proved not up to the task of keeping a lid on volatile London. In addition the troops he had taken had not been expecting a fight ( neither had Lambert or Monck, probably ) and with heavy turnover since 1651 there was little personal loyalty to Lambert and the chaotic conditions contributed to the lack of pay, supply, etc… So as Lambert tacked north, then south, bleeding troops and with the odds of armed conflict escalating ( as Monck’s chances of winning in a fight grew with every soldier who defected ), his army disintegrated.

Meanwhile Monck had been in solid control in Scotland as the only faction that mattered. He had purged his little army of all dissidents, kept it in fighting trim with officers personally loyal to him. Presumably his local control of resources and its smaller size allowed him to keep it paid and firmly under his control. Consequently when push came to shove he had every advantage as chaos reigned everywhere else.